Allow message aliases

[Biep]

In some situations it is handy to keep referring back to certain key messages in a room. For such cases it would be handy to be able to assign memorable aliases to such messages - akin to the way one can assign aliases to rooms. Supposedly the full alias for a message would include an alias for the room it is in.
This would also be useful in the context of Wiki rooms, as proposed before (#14645).

[Biep]

It would also allow automatically giving an alias to the first message in a room, thus allowing quick jumping to the room top.