Guest accounts should expire

This issue has been migrated from [#669](https://github.com/matrix-org/synapse/issues/669).

---

Currently, guest accounts will not expire, which leads to them cluttering up rooms, even though no one is using these accounts anymore.

Since the whole idea of guest accounts is that of a temporary life-span, this should be reflected in how they are handled server-side.

I propose that guest accounts automatically expire/leave all rooms they are in, if either:

a) The user goes offline

or

b) The account has no activity for 24 hours