compose: Fix behavior hitting enter with the preview area open.

The new behavior is:
(1) If enter-sends is enabled, just send the messsage.
(2) If enter-sends is not enabled, return focus to the compose area.

Based on great work by khantaalaman in #3673.

Fixes #3489.

static/js/compose.js

@@ -607,6 +607,17 @@ function send_message(request) {
     }
 }
 
+exports.enter_with_preview_open = function () {
+    clear_preview_area();
+    if (page_params.enter_sends) {
+        // If enter_sends is enabled, we just send the message
+        send_message();
+    } else {
+        // Otherwise, we return to the compose box and focus it
+        $("#new_message_content").focus();
+    }
+};
+
 exports.respond_to_message = function (opts) {
     var message;
     var msg_type;

static/js/hotkey.js

@@ -331,6 +331,12 @@ function process_hotkey(e) {
         return false;
     }
 
+    // There's special handling for when you're previewing a composition
+    if ($("#preview_message_area").is(":visible")) {
+        compose.enter_with_preview_open();
+        return true;
+    }
+
     // Shortcuts that don't require a message
     switch (event_name) {
         case 'narrow_private':