Give admin access on matrix side if their bridged users have admin access on xmpp side

[pravi]

Is your feature request related to a problem? Please describe.
When accessing a native xmpp room via matrix, the bridged room does not give admin access to any user on the matrix side (unlike IRC bridge, which gives moderator access to ops on irc side).

Describe the solution you'd like
Unlike the irc bridge, we can't automatically map admin users using their nickname, but if a bridged matrix user gets admin access on xmpp side, its corresponding matrix user can be given admin access on the matrix side.

Describe alternatives you've considered
Currently there is no way to give anyone on the matrix side admin access.

Additional context
Alternatively if we start the room on the matrix side, then since the bridge does not implement MAM, xmpp users will not get offline messages.

[pravi]

Related to #64