Shows membership events like join & leave that were previously hidden.
These events can be somewhat important to knowing what's going on in
the room (like if someone you want to respond to leaves), so hiding
them can leave users confused. A few things need to be changed
before this can be finalized.

TODO
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* [ ] Bundle room events under a dropdown like Polari (needs design)
* [ ] Change styling of room events (should they look like emotes? Should they use dim-label?)
* [ ] Decide if membership events should move rooms around like they do in Riot
* [ ] Change hardcoded strings to use gettext