Add support for emoji picker hotkeys in compose context.

This is a follow-up to merging the compose and reactions emoji
pickers.  The logic for what happens when the user picks an emoji via
the hotkeys (i.e. hits `enter`) was still attempting to add a reaction
to the currently selected message unconditionally.

This commit adds a check in the two `enter` key code paths, and does
the correct thing in each case.

Fixes #4736.

static/js/emoji_picker.js

@@ -182,6 +182,10 @@ exports.register_click_handlers = function () {
     });
 };
 
+exports.is_composition = function (emoji) {
+    return emoji.classList.contains('composition');
+};
+
 return exports;
 
 }());

static/js/hotkey.js

@@ -245,7 +245,11 @@ exports.process_enter_key = function (e) {
     }
 
     if (emoji_picker.reactions_popped()) {
+        if (emoji_picker.is_composition(e.target)) {
+            e.target.click();
+        } else {
             reactions.toggle_reaction(current_msg_list.selected_id());
+        }
         return true;
     }
 

static/js/reactions.js

@@ -116,9 +116,13 @@ function maybe_select_emoji(e) {
         e.preventDefault();
         var first_emoji = get_emoji_at_index(0);
         if (first_emoji) {
+            if (emoji_picker.is_composition(first_emoji)) {
+                first_emoji.click();
+            } else {
                 exports.toggle_reaction(current_msg_list.selected_id(), first_emoji.title);
             }
         }
+    }
 }
 
 $(document).on('click', '.emoji-popover-emoji.reaction', function () {