narrow: Add frontend support for is:dm narrow and /is/dm URL.

Adds support in the web app for `is` operator with the `dm` operand.
This will deprecate the `is` operator with the `private` operand,
but we keep support for backwards-compatibility with links/URLs.

This commit updates the web app default behaviors to default to
the new narrow/URLs `is:dm` and `/#narrow/is/dm` when navigating
and searching in the app.

There is some general clean up of references to private messages
or PMs to be either direct messages or DMs in these changes.

The general API changelog and documentation updates will be done
in a final commit in the series of commits that adds support for
the various new direct message narrows.

web/src/filter.js

@@ -91,7 +91,7 @@ function message_matches_search_term(message, operator, operand) {
 
         case "is":
             switch (operand) {
-                case "private":
+                case "dm":
                     return message.type === "private";
                 case "starred":
                     return message.starred;
@@ -228,6 +228,12 @@ export class Filter {
         operator = Filter.canonicalize_operator(operator);
 
         switch (operator) {
+            case "is":
+                // "is:private" was renamed to "is:dm"
+                if (operand === "private") {
+                    operand = "dm";
+                }
+                break;
             case "has":
                 // images -> image, etc.
                 operand = operand.replace(/s$/, "");
@@ -451,7 +457,7 @@ export class Filter {
 
         // All search/narrow term types, including negations, with the
         // property that if a message is in the view, then any other
-        // message sharing its recipient (stream/topic or private
+        // message sharing its recipient (stream/topic or direct
         // message recipient) must also be present in the view.
         const valid_term_types = new Set([
             "stream",
@@ -461,8 +467,8 @@ export class Filter {
             "pm-with",
             "group-pm-with",
             "not-group-pm-with",
-            "is-private",
-            "not-is-private",
+            "is-dm",
+            "not-is-dm",
             "is-resolved",
             "not-is-resolved",
             "in-home",
@@ -505,7 +511,7 @@ export class Filter {
             return true;
         }
 
-        if (_.isEqual(term_types, ["is-private"])) {
+        if (_.isEqual(term_types, ["is-dm"])) {
             return true;
         }
 
@@ -549,8 +555,8 @@ export class Filter {
     is_common_narrow() {
         // can_mark_messages_read tests the following filters:
         // stream, stream + topic,
-        // is: private, pm-with:,
-        // is: mentioned, is: resolved
+        // is:dm, pm-with:,
+        // is:mentioned, is:resolved
         if (this.can_mark_messages_read()) {
             return true;
         }
@@ -607,8 +613,8 @@ export class Filter {
                     return (
                         "/#narrow/stream/" + stream_data.name_to_slug(this.operands("stream")[0])
                     );
-                case "is-private":
-                    return "/#narrow/is/private";
+                case "is-dm":
+                    return "/#narrow/is/dm";
                 case "is-starred":
                     return "/#narrow/is/starred";
                 case "is-mentioned":
@@ -655,7 +661,7 @@ export class Filter {
                 }
                 context.zulip_icon = "hashtag";
                 break;
-            case "is-private":
+            case "is-dm":
                 context.icon = "envelope";
                 break;
             case "is-starred":
@@ -719,7 +725,7 @@ export class Filter {
                     return $t({defaultMessage: "Starred messages"});
                 case "is-mentioned":
                     return $t({defaultMessage: "Mentions"});
-                case "is-private":
+                case "is-dm":
                     return $t({defaultMessage: "Direct messages"});
                 case "is-resolved":
                     return $t({defaultMessage: "Topics marked as resolved"});
@@ -742,7 +748,7 @@ export class Filter {
 
     contains_only_private_messages() {
         return (
-            (this.has_operator("is") && this.operands("is")[0] === "private") ||
+            (this.has_operator("is") && this.operands("is")[0] === "dm") ||
             this.has_operator("pm-with") ||
             this.has_operator("group-pm-with")
         );
@@ -802,7 +808,7 @@ export class Filter {
             return terms;
         }
 
-        return terms.filter((term) => Filter.term_type(term) !== "is-private");
+        return terms.filter((term) => Filter.term_type(term) !== "is-dm");
     }
 
     _canonicalize_operators(operators_mixed_case) {
@@ -938,7 +944,7 @@ export class Filter {
             "id",
             "is-alerted",
             "is-mentioned",
-            "is-private",
+            "is-dm",
             "is-starred",
             "is-unread",
             "is-resolved",
@@ -1024,6 +1030,7 @@ export class Filter {
                 return verb + operand + " messages";
             case "mentioned":
                 return verb + "@-mentions";
+            case "dm":
             case "private":
                 return verb + "direct messages";
             case "resolved":