Deactivate/delete student accounts

[Víctor López Juan]

Allow teachers and/or students to deactivate accounts, so that they do not show up in the list of students (or show up greyed out), and cannot submit. This also excludes them from functionality like "Mail all students in the course".

Use cases:

• A student drops a course, and does not want to receive e-mails from the "Mail all students feature".
• A teacher wants to ensure that a student shows up on fire iff this student is registered in the course.

Functionality:

• A student can deactivate their account any time.
• A student can activate their account any time, unless the course is closed for registrations.
• A teacher can activate or deactivate student accounts at any time.

Also, to complement this functionality: * A teacher can remove pending email verifications from the UI.

Note that all the data from the student (submissions, group membership…) is preserved upon deactivation. So if the student is part of a group, the teacher should take them out before deactivating them. Also, if a student is part of a group, one of the other members of the group could submit for them, even if their account is deactivated.