Allow admin to assign a user to a different role

[amellie]

Dependency

This issue depends on #19

Story

 
As an admin
I Want to assign a different role to an existing user
So That the user can perform tasks specific to the role (e.g. marking, etc)
 

Acceptance Criteria

 

• Add a new admin user into the database with the role = ADMIN
• We should have a separate admin page (only accessible by users with ADMIN role) which allows the admin to assign a role to a user
• At the very minimum, the page should show: the list of users, type of role (ADMIN, TEACHER, or STUDENT), and a button to submit
• If the user is a teacher and the role selected is also a teacher, then an error message should be shown: "The user XXX already has the selected role"