Consider holidays for next schoolday

[ImGxrke]

Fixes #1179

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[Jonas-Sander]

Hey, thanks! :) My development environment is currently kinda broken, I'll try to review it in the next few days.

Several tests are currently failing with the following error: Exception: A BlocProvider ancestor with Type BlocProvider<HolidayBloc> should be given in the widget tree. This means that in the setup code for the widget tests you'll need to add a BlocProvider<HolidayBloc> to the widget tree in the test file homework_dialog_test.dart line 281.
There are other failing unit tests as well though.

[ImGxrke]

The test are now fixed, however I am not sure if the implementation of the BlocProvider<HolidayBloc> is done in the right way.

[Jonas-Sander]

LGTM! Good job :)