[C2] Enable roles combination

[ywarnier]

The structure in C2 allows us to assign several roles to the same user. However, the interface to do that is still missing.
Change the dropdown box in the user edition page to allow for a multiple-select. Make it 8 rows high to make sure we see most of the roles in one single step.

Adapt the api_is_allowed_to_edit() function to use the method developed in #5644. The permissions should be the final result of checking for permissions by all available roles on the item we're looking at. Whenever the edition is just about an item in the course where the teacher is assigned, make sure you bypass all this. Permissions are only important for (few) specific items inside courses and most items outside the courses (skills, badges, sessions, users, settings, etc).

Maybe have a static list of things to check for inside the api_is_allowed_to_edit() function and check it quickly before going deeper to api_get_permission()?