Upgrade to Vue 3 when it becomes more stable.

One of the most notable features of vue3 is proper portal/teleport support which doesn't re-render portal contents on each change. Another is Composition API which we can hopefully use to get rid of HOCs.

There's an exploratory branch "vue3" with some initial attempts to use vue3.

vue-chat-scroll dependency could be problematic and we might need to rewrite it...