Stop using django-multiselect

[hmpf]

It seems to be dead and the forks all seem dead.

We currently use it only for TimeRecurrence.days. We used to use it for NotificationProfile.media.

1. We'd have to first get rid of the usage
2. make a migration for that
3. in a squashing of the migrations of notificationprofile replace all migrations mentioning the library
4. then in a final migration delete the migration-files importing the field.

Three releases: patch (or minor), patch, minor (or major), the final one drops the dependency.

[johannaengland]

Closed by #711 and #724.