Dedicated admin API for blocking a room

[DMRobertson]

#11228 was confusing to understand, because the API call to shutdown room does several different things at once. In that change I ended up introducing a bodge, allowing admins to use that endpoint to block a room that doesn't exist.

I think there is a case for an admin API call that blocks this room and nothing else. (See also #2392).

[dklimpel]

IMO unblocking a room would be helpful, too.
Something like:

POST /_synapse/admin/v1/rooms/<room_id>/block
{
    "block": true
}

And perhaps a list of blocked rooms:
GET /_synapse/admin/v1/rooms/blocked

[DMRobertson]

Closed via #11324 .