List in the settings the users having commit rights on the project
Add a form to add a user
Add method to the internal API to add an user to a project
Add template to add an user
Add a new endpoint to add a new user to a project

Which basically gives the specified user commit rights on the project
directly
Add link in the setting page to add an user to a project
Adjust the CSS to make things prettier
Specify the owner of the project in the user list on the setting page
Add logic to the repo controller to remove someone from a project
For each user associated with a project, offer the possibility to remove it
Small CSS change to make things prettier
Adjust a little the error messages returned when the user is not allowed