More Robust Role Assignment

[YashoSharma]

Currently there is no endpoint to allow users to assign other users roles, we want to eliminate manual role assignment by creating an endpoint which:

• Allows any NON_PROFESSIONAL or above user to modify the roles of other users
• A user should only be able to assign or remove a role equivalent to their role or below
  -e.g. A VOLUNTEER should not be able to assign a SUPERUSER role to a user
• Users should not be allowed to modify the roles of users who have a role higher than their own
• Users should not be allowed to assign staff roles to attendees, or mentors
• Users cannot use this functionality if they are using the user impersonation utility
• Users can't modify their own roles