Untag yourself as product collaborator

[JonathanGarro]

Users can tag themselves as collaborators on products that have been posted to the portal, but there currently isn't a way to untag yourself. The route to assign yourself currently:

• Checks that you are not the original poster
• Indexes all user IDs associated with the product's collaborator_ids column and loops over it with the user ID of the person trying to add themselves
• Appends the ID if not already present.

A route to remove an ID from that list would need to run a similar validation, then run remove() on the list.

[JonathanGarro]

I've added a remove_supporter_from_product route and adjusted the view so that users can both tag and remove themselves. There was an error that this route introduced when removing someone, as the product page does validation on whether the collaborator_ids column is None. So I added a conditional that inserts 0 into the column if removing a user leaves the column blank.