Make agreement file not blank when everyone has an agreement

[dakaza98]

Currently all agreement files are allowed to be blank since there are people who have signed the old agreement when the system was created. Instead of having them resign their agreements, those agreements will continue to live in their folder on klubbmästarens desk. Therefor the field is blank. When every user has an agreement file, blank=True can be removed.

The agreement_file field can also be removed from the AgreementForm in forms.py

Ask the klubbmästare if all the old agreements in their folder in their desk have been removed and check in the admin pages on the production server if all users have an attached agreement