Support informational overlays in hashchange system.

This helps the !modal_link links in our tutorial messages.

Fixes #5206.

static/js/hashchange.js

@@ -149,6 +149,15 @@ function do_hashchange(from_reload) {
     case "#streams":
         ui_util.change_tab_to("#streams");
         break;
+    case "#keyboard-shortcuts":
+        ui.show_info_overlay("keyboard-shortcuts");
+        break;
+    case "#markdown-help":
+        ui.show_info_overlay("markdown-help");
+        break;
+    case "#search-operators":
+        ui.show_info_overlay("search-operators");
+        break;
     case "#drafts":
         ui_util.change_tab_to("#drafts");
         break;

static/js/ui.js

@@ -145,7 +145,9 @@ exports.show_info_overlay = function (target) {
         overlays.open_overlay({
             name:  'informationalOverlays',
             overlay: overlay,
-            on_close: function () {},
+            on_close: function () {
+                hashchange.changehash("");
+            },
         });
     }