User display names and avatars are incorrect wrt. room state

[Ralith]

Nheko currently keeps a global map of user IDs to display names. In addition to being a safety hazard, this causes the incorrect display name to be looked up for historical messages, e.g. if a user just changed it, and will give incorrect behavior if a user has different display names per-room. Display names should be tracked like any other room state, and displayed in events according to the state of the room when the event occurred.

[Ralith]

It looks like avatars have the same bug.

[ollieparanoid]

Possibly related: #113