Add ability to deactivate user.

[karbassi]

For admin only.

There should be a way to set the is_active flag for a specific guardian without having to manually edit the records inside the django admin.

Tasks

• [ ] Create a page user the admin url structure. e.g. /admin/user/deactivate.
• [ ] This page must only allow admins to access this page.
• [ ] Allow the admin to search for users that are active. Search must allow searching via email or first/last name.
• [ ] Once found, allow the admin to 'deactivate' the user. This should set the is_active on the user Model and the Guardian/Mentor models.