Log the end of collegist status

[viktorcsimma]

We should know when users became alumni, to be able to calculate membership counts in different years.

An option would be to simply add a timestamp to the role_users table; another would be to add a field to the educational informations.

[kdmnk]

Add it to the educational_information, our roles are complex enough. Should be set automatically when we deactivate them.

[kdmnk]

Also include:

• ensure that we give statuses only to active collegists

[viktorcsimma]

Probably should be merged with #392.