Prompt user to fork project when opening editor without edit permissions

When a user wants to edit a project but does not have permissions, they should be prompted to fork the project before being able to make changes that they will not be able to save.

Currently the multi-repo editor does not perform any checks before allowing edit mode to be entered. Upon trying to save an error is shown: An error occurred while committing your changes

Design

• Replace current "edit" inline fork bar
• Always summon up this modal if the user does not have enough permissions.