Have tag operations dialog reflect time machine configuration

If a tag deletion is going to remove images, but those images are not going to be removed immediately, we need to message that

Fixes https://www.pivotaltracker.com/story/show/143063199

endpoints/api/repository.py

@@ -379,6 +379,7 @@ class Repository(RepositoryParamResource):
       'is_starred': is_starred,
       'status_token': repo.badge_token if not is_public else '',
       'trust_enabled': bool(features.SIGNING) and repo.trust_enabled,
+      'tag_expiration_s': repo.namespace_user.removed_tag_expiration_s,
     }
 
     if stats is not None: