Add support for different renderings to topic events

[Johennes]

This extends the m.room.topic event with a new m.topic event that uses the same structure as the m.message event on room messages, thereby allowing for different renderings of room topics.

This largely just copies and renames things from message events. I didn't put any effort into extracting common logic between the two events at this point yet to not over-complicate things.

Marking as draft while I try to actually use this elsewhere.

Relates to: element-hq/element-web#5180
MSC: matrix-org/matrix-spec-proposals#3765

[ShadowJonathan]

Does the spec allow this? If not, shouldn't this need an MSC as well?

[Johennes]

Does the spec allow this? If not, shouldn't this need an MSC as well?

I was unsure as well when starting but @turt2live said it wouldn't. Essentially this is just using the m.message content defined in MSC1767.

[Johennes]

I was unsure as well when starting but @turt2live said it wouldn't.

Ok, looks like this was a misunderstanding on my part. I'll put in a dedicated MSC.

[turt2live]

To answer the question though: yes, the spec does allow this to happen. Extensions to events are entirely permitted if a client/project wants to use them, though typically they're accompanied by an MSC.

Implementation before MSC is also entirely valid.

[Johennes]

MSC3765 has been created. As discussed separately, this is still not intended to be merged yet in order not to complicate future architectural changes.