events: Add unstable support for reference relations

[zecakeh]

According to MSC3267

Working towards #1031

[jplatte]

Hm, that MSC is currently in draft status and mentions threads as a motivation, which seem to have been implemented differently in the meantime. Are you sure it's useful to have the MSC in Ruma in its current state?

[zecakeh]

Yes, it's needed for polls

[jplatte]

I'm not convinced it should be its own feature flag, given it has literally no semantics on its own. I've commented on the MSC, let's see what others think.

[zecakeh]

The reason I put a feature flag is that there might be another unstable MSC that uses it before polls are made stable. I don't mind not making it separate.

As a side note, in-room key verification events seem to use it too, although it's not defined as being the same type.

[zecakeh]

I'm closing this as the MSC has been updated to reflect that this type has no meaning on its own, so it cannot be applied on any message event, as opposed to the other relations.

It would still be nice to move relations as suggested in #1121, that way we could use the same Reference type that's currently in room::encrypted for polls.