Rooms should have the option of kicking out lurkers (SYN-8)

This issue has been migrated from [#1202](https://github.com/matrix-org/synapse/issues/1202).

---

#matrix:matrix.org is full of lots of lurkers who have joined once and don't look to be coming back anytime soon. It would be lovely to have a setting on the room to boot them out to stop bloating the membershiplist.

One way of doing this is to run a bot as a privileged user on the channel to issue kick events to the right people.

This is a bit kludgy though - it would be nicer if the HS for the highest-power user on a channel could go and automatically issue the appropriate events to clear things up.

(Unclear how this works with e2e encryption)

(Imported from https://matrix.org/jira/browse/SYN-8)

(Reported by @ara4n)