Implement archived rooms

[kegsay]

Specifically this is the include_leave part of the filter.
Sytests:

    × Archived rooms only contain history from before the user left

[alexchan150]

I would like to get started on this!

[alexchan150]

I'm struggling a bit after going through the sync logic and specs - can't seem to find any references to the roomFilter in the codebase. Is there any more information on the include_leave part and how that pertains to this issue?

[kegsay]

When you leave a room, you no longer get updates to the room, or in fact told about the room at all, via /sync unless you give /sync a filter with include_leave: true. Doing this will cause /sync to return information on left rooms (archived rooms)

[MadLittleMods]

I've removed good first issue as this problem has turned out pretty hard, #1671

• Sytest is hard to work with, figure out what is wrong, and then reason about is that even correct or just Synapse behavior
• The CompleteSync/IncrementalSync code is hard to reason about
• Needs type filtering for events -> Basic sync filtering #1721
• Might require better/full history visibility implementation