Fix facepile display issues

Fixes https://github.com/vector-im/element-call/issues/434 and a
separate bug where the facepile would just disappear off to the left
(because we kept increasing the size even though we capped the number
of circles at 8 plus the overflow one).

src/room/PTTCallView.module.css

@@ -28,6 +28,7 @@
   display: flex;
   flex-direction: column;
   margin: 20px;
+  text-align: center;
 }
 
 .participants > p {