Fix duplicate emoji entries in compose emoji picker.

We were incorrectly appending all the emoji into the emoji picker
every time it was opened, rather than just once.

Note by tabbott: Arguably this isn't the right fix, in that it might
be better to just render the emoji picker once at the beginning.  But
this definitely fixes the bug.

Fixes #3952.

static/js/popovers.js

@@ -358,7 +358,7 @@ function render_emoji_popover() {
             realm_emoji: emoji.realm_emojis,
         });
     }());
-
+    $('.emoji_popover').empty();
     $('.emoji_popover').append(content);
 
     $('.drag').show();