[BUG] students can join different class, but use only the last one

[hasan-sh]

Describe the bug
Currently, students can be in different classes, but the customization of the last class is always applied. This issue should tackle the following:

• allow inviting a student to different classes at the same time
  • now, if you invite student1 to a Class1, and then you (or some other teachers) invite them to Class2, the invitation will fail because there's one invitation for student1.
  • allow multiple invitations.
• allow students to switch between classes they are in
  • in the menu bar show dropdown with all classes (or in the profile setting section)
  • and students can choose which one to use.

[Felienne]

To be honest, I am not sure for what scenarios this is useful, so I will do a bit of inquiry with tecchers first. I know some teachers wanted to use this as a "hack" for customizations per student, but it would be better to truly support that!

Let's first make sure the state cannot happen, by giving a better error messages with a second invite and an error message with a second class joining (see #4811)