Resolve "Prevent teachers from removing their own rights"

When using "Add a single participant option", a teacher can't demote themselves if they are the only teacher in the edition, they can demote a teacher with a warning if there are more than one for the edition.

The leave button in the participants page is inaccessible for the teacher if there is only one teacher for the edition.

Also does not let users demote using Csv file.

Closes #621 (closed)