Render and allow pinning of events

`m.room.pinned_events` are a [specced](https://spec.matrix.org/unstable/client-server-api/#mroompinned_events) state event type which allows users in a room to pin events they'd like to keep readily accessible.

Element Web seems to be making a push towards enabling this feature by default (tracking issue https://github.com/vector-im/element-meta/issues/1624), and it would be nice to have it on iOS as well.