Consider replacing `org.matrix.mjolnir.watched_lists` with a state event.

The main purpose of this is to alleviate concerns about centralization / single point of failure etc. Since now this information is a state event in a space or a protected room or something that is to be decided, then you can have multiple mjolnirs synchronizing policies without conflict or need to make sure their account data are exactly the same. This also helps a little with migrating accounts. 