Release milestone 5.1

[StianJul]

Milestone 5.1 can not be merged into master before the following issues are resolved

We can still release rc-versions from the milestone-5.1 branch.

Django-allauth

• [x] https://github.com/devilry/devilry-django/issues/1156

Possible breaking changes has been made to Django-allauth versions compatible with Django 3/Python 3, and must be tested before a proper release.

[StianJul]

https://devilry.readthedocs.io/en/master/sysadmin/migrationguides/migrate-from-5.0.1-to-5.1.0.html