Add better admin options to clean up portal rooms and manage users

[maranda]

It's currently very hard to manage user subscriptions to the bridge, this is quite requirement to do basic operating of the bridge on any public service instance as often users become inactive being totally unaware and forgetting that the bridge keeps sync'ing data from their WA for example. It's a burden for HomeServer administration where the only way to deal with these issues is database surgery and continuos restarting of the bridge.