Add command to create rooms to become creator (room v12 feature)

[sophie-h]

If rooms are created as v12, the creator has a special role. I think in many cases it would make sense to make mjolnir the creator. Therefore, it would be useful if there was a command like

!mjolnir rooms createa <room alias>

or don't specify an alias and print the room id.

I think all other configuration can be done manually afterward.