JS: Results/marking view + annotations

This PR includes JS updates for files relevant to the results/marking view. This includes the SourceCodeGlower, used for displaying the code and for highlighting the annotated lines, and the file picker (the <-, -> buttons and the select dropdown).

Prototype has been converted to jQuery or vanilla JS.

This includes changes/fixes for PR https://github.com/MarkUsProject/Markus/pull/1244.
Actualy, I think we are in `en.yml`. It's fairly inconsistent.