Make namespace deletion asynchronous

Instead of deleting a namespace synchronously as before, we now mark the namespace for deletion, disable it, and rename it. A worker then comes along and deletes the namespace in the background. This results in a *significantly* better user experience, as the namespace deletion operation now "completes" in under a second, where before it could take 10s of minutes at the worse.

Fixes https://jira.coreos.com/browse/QUAY-838

data/database.py

@@ -470,13 +470,21 @@ class User(BaseModel):
                                  RepositoryNotification, OAuthAuthorizationCode,
                                  RepositoryActionCount, TagManifestLabel, Tag,
                                  ManifestLabel, BlobUploading, TeamSync,
-                                 RepositorySearchScore} | beta_classes
+                                 RepositorySearchScore, DeletedNamespace} | beta_classes
       delete_instance_filtered(self, User, delete_nullable, skip_transitive_deletes)
 
 
 Namespace = User.alias()
 
 
+class DeletedNamespace(BaseModel):
+  namespace = QuayUserField(index=True, allows_robots=False, unique=True)
+  marked = DateTimeField(default=datetime.now)
+  original_username = CharField(index=True)
+  original_email = CharField(index=True)
+  queue_id = CharField(null=True, index=True)
+
+
 class UserPromptTypes(object):
   CONFIRM_USERNAME = 'confirm_username'
   ENTER_NAME = 'enter_name'