Submit model form assumes the user owns the project repository

[lehors]

The form to submit a new model asks the user to select the project repository out of a list of repositories taken from the user's account. The list is incomplete and in practice the model repository a user creates a record for is typically not owned by the user.

So, the repository should simply be a text field the user can use to enter any repositories (including outside GitHub).

The existing code that retrieves information from the repository could still be used if any such information is available but skipped if that's not possible.