Add last_accessed information to User and expose for robot accounts

Fixes https://jira.coreos.com/browse/QUAY-848

data/database.py

@@ -440,8 +440,8 @@ class User(BaseModel):
   location = CharField(null=True)
 
   maximum_queued_builds_count = IntegerField(null=True)
-
   creation_date = DateTimeField(default=datetime.utcnow, null=True)
+  last_accessed = DateTimeField(null=True, index=True)
 
   def delete_instance(self, recursive=False, delete_nullable=False):
     # If we are deleting a robot account, only execute the subset of queries necessary.