Refactor - Authority roles should be maintained in an Enum

Description

We currently have the following roles: ROLE_ADMIN, ROLE_TEACHER, ROLE_TA, ROLE_STUDENT

Since we have a fixed set of values, we should replace these strings with an enum.

Priority - Medium

This would make the could more maintainable and would reduce the chance that typing errors are introduced

Definition of done

When roles have been replaced with enums.