Implement permission levels though out the application

[deeppunster]

• Only login page does not require the LoginMixIn
• Each page will verify if the user is allowed to access that page
  • Throws a polite error message and allow return to previous page if not
• So far, only the Add user page and the change user permission page will have conditional logic based on the permission of the user
• Perhaps this can be isolated to one class or function that contains the necessary logic