Lazy-load emoji picker in post form

When clicking the reply button, we used to load the whole emoji picker.
This causes a considerable delay even if the user is not going to use
the emoji picker. Now the content of the emoji picker is loaded only
after the user has explicitly opened the emoji picker.

Ref: grouped-emoji-picker

src/components/emoji_input/emoji_input.vue

@@ -19,6 +19,7 @@
         v-if="enableEmojiPicker"
         ref="picker"
         :class="{ hide: !showPicker }"
+        :showing="showPicker"
         :enable-sticker-picker="enableStickerPicker"
         class="emoji-picker-panel"
         @emoji="insert"