Allow multiple recipient objects for one announcement

Closes #188

See merge request AlekSIS!166

Remove merge migration 0014 and move 0013_multiple_recipients_announcement to 0014_multiple_recipients_announcement instead

# Conflicts:
#	aleksis/core/models.py

Fix some mistakes in app config system check

See merge request AlekSIS!164

Mandate use of AlekSIS base model

Closes #182

See merge request AlekSIS!163

add dashboard into main menu

Closes #189

See merge request AlekSIS!165

close #189

There is no reason we should not get a warning when accidentally
breaking the core, is there ;)?

Improves readability by declaring that this filter is not part of the
system check, but limits the system check to a subset of apps.

Mark DashboardWidget as a pure Django model

Closes #182.

Advances #182.

Advances #182

lambda expressions cannot be used as field defaults because they cannot
be serialised in migrations.

Remove created_at field from core models.

Advances #182.

Enable django sites framework

See merge request AlekSIS!162

Show announcements on dashboard

Closes #187

See merge request AlekSIS!161

# Conflicts:
#	aleksis/core/models.py

Improve announcement model

See merge request AlekSIS!160

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

- 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