Add proper role mangement

[rzo1]

The application was originally intended to be used only for the initial password rollout, but is now in permanent use. This is why there is no permissions management or role model implemented yet.

At the moment, any helpdesk employee can reset any AD account. So far, we have taken organisational measures to prevent misuse by only allowing selected employees to access the application and by keeping an audit log.

Proposal:

• Implement a role model function, so that, for example, IT staff in remote locations can reset the passwords of students in their respective faculties but not of their counter-parts at different locations.
• Further details of the role model need to be discussed.