Add type hints. Closes #20.

apps/official/BiscuIT-App-Alsijil

apps/official/BiscuIT-App-Chronos

apps/official/BiscuIT-App-Exlibris

apps/official/BiscuIT-App-SchILD-NRW

apps/official/BiscuIT-App-Untis

biscuit/core/apps.py

biscuit/core/decorators.py

biscuit/core/mixins.py

biscuit/core/models.py

biscuit/core/tables.py

biscuit/core/templatetags/data_helpers.py

biscuit/core/util/core_helpers.py

biscuit/core/util/messages.py

biscuit/core/util/sass_helpers.py

biscuit/core/views.py