Add delete room admin endpoint (#7613)

The Delete Room admin API allows server admins to remove rooms from server
and block these rooms.
`DELETE /_synapse/admin/v1/rooms/<room_id>`
It is a combination and improvement of "[Shutdown room](https://github.com/matrix-org/synapse/blob/develop/docs/admin_api/shutdown_room.md)" and "[Purge room](https://github.com/matrix-org/synapse/blob/develop/docs/admin_api/purge_room.md)" API.

Fixes: #6425 

It also fixes a bug in [synapse/storage/data_stores/main/room.py](synapse/storage/data_stores/main/room.py) in ` get_room_with_stats`.
It should return `None` if the room is unknown. But it returns an `IndexError`.
901b1fa561/synapse/storage/data_stores/main/room.py (L99-L105)

Related to:
- #5575
- https://github.com/Awesome-Technologies/synapse-admin/issues/17

Signed-off-by: Dirk Klimpel dirk@klimpel.org

tests/storage/test_room.py

@@ -55,6 +55,10 @@ class RoomStoreTestCase(unittest.TestCase):
             (yield self.store.get_room(self.room.to_string())),
         )
 
+    @defer.inlineCallbacks
+    def test_get_room_unknown_room(self):
+        self.assertIsNone((yield self.store.get_room("!uknown:test")),)
+
     @defer.inlineCallbacks
     def test_get_room_with_stats(self):
         self.assertDictContainsSubset(
@@ -66,6 +70,10 @@ class RoomStoreTestCase(unittest.TestCase):
             (yield self.store.get_room_with_stats(self.room.to_string())),
         )
 
+    @defer.inlineCallbacks
+    def test_get_room_with_stats_unknown_room(self):
+        self.assertIsNone((yield self.store.get_room_with_stats("!uknown:test")),)
+
 
 class RoomEventsStoreTestCase(unittest.TestCase):
     @defer.inlineCallbacks