[Glitch] Fix stacking order of emoji picker

Port 8a378d4c3d to glitch-soc

Signed-off-by: Thibaut Girka <thib@sitedethib.com>

app/javascript/flavours/glitch/styles/components/emoji.scss

@@ -17,6 +17,7 @@
   box-shadow: 4px 4px 6px rgba($base-shadow-color, 0.4);
   border-radius: 4px;
   margin-top: 5px;
+  z-index: 2;
 
   .emoji-mart-scroll {
     transition: opacity 200ms ease;