bug: API does not return a mentions field when a message is edited to contain a mention about backend

[insertish]

This is intentional behaviour as the 'mentions' field is meant to be interpreted as the users that the message notified at the time of sending. Calculating the current mentions list should be relatively cheap for the receiving client if necessary.

I'm going to close this out unless if others chime in and agree this should be changed, I would probably recommend proposing this change as an RFC for further discussion if there is interest.