Add a bulk user info endpoint and deprecate the old one (#46)

The current `/user/<user_id>/info` API was useful in that it could be used by any user to lookup whether another user was deactivate or expired. However, it was impractical as it only allowed for a single lookup at once. Clients trying to use this API were met with speed issues as they tried to query this information for all users in a room.

This PR adds an equivalent CS and Federation API that takes a list of user IDs, and returning a mapping from user ID to info dictionary.

Note that the federation in this PR was a bit trickier than in the original #12 as we can no longer use a federation query, as those don't allow for JSON bodies - which we require to pass a list of user IDs. Instead we do the whole thing of adding a method to transport/client and transport/server.

This PR also adds unittests. The earlier PR used Sytest, presumably for testing across federation, but as this is Synapse-specific that felt a little gross. Unit tests for the deprecated endpoint have not been added.

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