Features:
- Add settings activity, also containing "About" information
- Add option to switch between light and dark theme in settings
- Add a button to the drawer to view topics from all subjects at once
- Switch between showing everything and showing only topics with an assignment in topics activity

Improvements:
- Show subjects in main activity before their marks have been loaded, and open topics activity when a subject with no marks is clicked
- Improve layout of topic list items by adding some fancy icons and better detect descriptions wrongly written in the title field and viceversa
- Replace some icons and make sort button a switch in marks activity
- Cache extracted data to prevent reloading it
- Center marks if they don't fill the screen horizontally in subject activity

Bugs fixed:
- Use older version of OkHttp to fix support on API 14+ (Android 4.0+)
- Fix dates were not always considered to be in English, making parsing fail sometimes
- Correctly detect network errors, and mark other errors as unknown
- Migrate to AndroidX to better support old Android versions all the way up to Android 10
