Ability to change history visibility retrospectively. (SPEC-210)

### Background

The IRC bridge provisioned rooms without the `m.room.history_visibility` flag set. This was a mistake and needed to be rectified. However, `m.room.history_visibility` doesn't allow retroactively changing the visibility of past events.

The solutions available at the time were:
- Redact every message.
- Manually delete old history from the room.
- Change the matrix room the IRC bridged to and seal the old room.
### Problem

We don't currently have a way to retroactively say either:
- users can only see events if they were joined at the time
- new users can't see history from before a given event.

(Imported from https://matrix.org/jira/browse/SPEC-210)

(Reported by @erikjohnston)

No files, tests, or entry points are named. Start by locating the specification for m.room.history_visibility and compare its current behavior with the two retrospective-visibility cases described here. Done means the required protocol behavior and scope are agreed and documented well enough for implementation.

Written by the indexing model from the issue text.