Role assignment

[shreychowdhary]

Fixes issue #108

• 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
• Users should not be allowed to modify the roles of users who have a role higher than their own
• Users can't modify their own roles
• Users cannot use this functionality using user impersonation
• Users should not be allowed to assign staff roles to attendees, or mentors
• Testing

[YashoSharma]

I think a lot of this functionality should be an extension of the user routes we already have.