Add Co-Ownership UI

[theory]

Need a UI for users to login and assign co-ownership of extensions to other users. They should also be able to transfer ownership to other users. The database interface is already there in the grant_coownership(), revoke_coownership(), and transfer_ownership() functions.

N.B.: Figure out why grant_coownership() doen't appear in the DB API wiki page.

[strk]

@theory do you have an estimate of the effort required to make this support available ?

[theory]

Not a ton. The database stuff is there; it would need routes in the router, controller methods and templates for the views for it. The UX would need to be designed to allow a user to be added as a co-owner, probably by implementing a search on the users table. From there, the controller would use the database ownership functions to execute the changes.