Build MediaRepositoryResource as a homeserver dependency

This avoids the scenario where we have four different PreviewUrlResources
configured on a single app, each of which have their own caches and cache
clearing jobs.

synapse/server.py

@@ -60,7 +60,10 @@ from synapse.http.matrixfederationclient import MatrixFederationHttpClient
 from synapse.notifier import Notifier
 from synapse.push.action_generator import ActionGenerator
 from synapse.push.pusherpool import PusherPool
-from synapse.rest.media.v1.media_repository import MediaRepository
+from synapse.rest.media.v1.media_repository import (
+    MediaRepository,
+    MediaRepositoryResource,
+)
 from synapse.state import StateHandler
 from synapse.storage import DataStore
 from synapse.streams.events import EventSources
@@ -121,6 +124,7 @@ class HomeServer(object):
         'http_client_context_factory',
         'simple_http_client',
         'media_repository',
+        'media_repository_resource',
         'federation_transport_client',
         'federation_sender',
         'receipts_handler',
@@ -293,6 +297,11 @@ class HomeServer(object):
             **self.db_config.get("args", {})
         )
 
+    def build_media_repository_resource(self):
+        # build the media repo resource. This indirects through the HomeServer
+        # to ensure that we only have a single instance of
+        return MediaRepositoryResource(self)
+
     def build_media_repository(self):
         return MediaRepository(self)