Move uploading action before the QR action.
[REBASE] Always finish the action mode on click.
Use dark theme for player controls.

Currently uses the wrong accent colour since there’s no dark variant of
the Ach so! theme yet.
Fix some action mode quirks.

- Action mode would lose the selection but stay active after onPause().
- Selecting items using a long press would not work after the initial
one.
Remove OverlayCheckableLayout.

It's easier and more flexible to implement manually.
Use a more descriptive name for the data observer.
Check if upload error message is null.
[REBASE] Temporarily disable choosing videos.
Keep our local thumbnails for now.
Merge branch 'improvement/LL-831' into review

Conflicts:
	app/src/main/java/fi/aalto/legroup/achso/adapter/VideoGridAdapter.java
Merge branch 'review' of https://github.com/learning-layers/AchSo into review