Move RoomContextHandler out of Handlers

This is in preparation for moving GET /context/ to a worker

synapse/handlers/room.py

@@ -395,7 +395,11 @@ class RoomCreationHandler(BaseHandler):
             )
 
 
-class RoomContextHandler(BaseHandler):
+class RoomContextHandler(object):
+    def __init__(self, hs):
+        self.hs = hs
+        self.store = hs.get_datastore()
+
     @defer.inlineCallbacks
     def get_event_context(self, user, room_id, event_id, limit):
         """Retrieves events, pagination tokens and state around a given event