Offline cache

Implement offline caching of homework and the ability for the user to let the data be automatically updated twice a day.

* Homework is only downloaded again when the user views it, if the stored homework is older than 1.5 hours
* Filtering the homework after date is now done in the app instead of on the server to support offline caching
* If the setting is enabled, homework is downloaded in the background at 14:15 and 17:15 every day