Manager permissions - fixes #40

Fixes #40 (closed)

This MR updates permissions for Managers to no longer be able to add and remove teachers.

To implement this, two new permission checks (2xcanUpdateParticipant) were added and called upon removing and creating participants.

Additionally, the remove button is now no longer visible for teachers whenever the user doesn't have the rights to remove them and the option to add teachers in the participant form is no longer available to managers.