State event to describe bridge constraints for messages

We currently have no way to indicate to a user how "well" their message will bridge through to a protocol. Factors like support for replies, images, or even markdown might not exist on a bridge and we need a way to tell the user that their message will be sent in a lesser form.

This also ties into the complaints that the IRC bridge [does nothing to limit the size of messages](https://github.com/matrix-org/matrix-appservice-irc/issues/701) which tends to piss off some channel ops, understandably.

I'd propose a `m.bridge.constraints` state event keyed off the bridge's userid. The intention would be to hint to the client that their messages may be sent incomplete in some way. However, this would not stop the user from sending messages and bridges should still be prepared to handle them.

No file, test, or entry point is named. Start by reviewing the Matrix specification's existing state-event conventions and the linked IRC bridge size-limit issue. Done should mean the proposed constraint event's scope and behavior are settled and documented clearly enough for clients and bridges to implement.

Written by the indexing model from the issue text.