Improve announcement model

See merge request AlekSIS!160

- Add __str__
- Add verbose names
- Make description optional
- Add new default for valid_until

https://github.com/Bouke/django-two-factor-auth/issues/335#issuecomment-584180803

Add way for dashboard widgets to add custom CSS/JS files which will be added in the html head

See merge request AlekSIS!155

Allow getting announcements for a set of objects

See merge request AlekSIS!157

Add announcement model

Closes #166

See merge request AlekSIS!156

Closes #166.

Maintain default data and implement app hooks

See merge request AlekSIS!154

Currently translated at 62.8% (98 of 156 strings)

Translation: AlekSIS/AlekSIS
Translate-URL: https://translate.edugit.org/projects/aleksis/aleksis/de/

This change is primarily intended to allow models to declare their own
default data, without having to use data migrations. Data migrations
have the downside of being run only once, so e.g. `manage.py flush` will
delete default data wchich will never be restored. Also, I consider it
more natural todefine default data setup on model classes.

In order to achieve that, this adds a generic interface for apps to
register hooks called on several signals, by simply implementing methods
with the same name on their AppConfig instance.

Add custom templates for error messages in forms generated by django-material

See merge request AlekSIS/AlekSIS!147

Make primary group optional

See merge request AlekSIS/AlekSIS!152