Add configurable limits for number of builds allowed under a namespace

We also support that limit being increased automatically once a successful billing charge has gone through

data/database.py

@@ -439,6 +439,8 @@ class User(BaseModel):
   company = CharField(null=True)
   location = CharField(null=True)
 
+  maximum_queued_builds_count = IntegerField(null=True)
+
   def delete_instance(self, recursive=False, delete_nullable=False):
     # If we are deleting a robot account, only execute the subset of queries necessary.
     if self.robot: