Per-room overwrites of settings

**Is your suggestion related to a problem? Please describe.**
In very large rooms (or rooms for announcements) it is sometimes best to disable the `Show join/leave messages (invites/kicks/bans unaffected)` or `Show display name changes` or other settings which seem relevant per-channel. The downside though is that this has a global effect across all rooms.

**Describe the solution you'd like**
The ability to have per-room overwrites would solve this problem. The per-room settings should override the global ones (the ones now present in the preferences pane).

Start by reviewing the existing global settings in the preferences pane and how room settings are represented. Determine which listed settings need room-level overrides and how those overrides should take precedence over global values. Done means the requested settings can be configured per room without changing behavior in other rooms.

Written by the indexing model from the issue text.