[FEATURE] Support submissions outside course

[pdawyndt]

Plugin only recognizes comments with URL for course-scoped exercises like:

https://dodona.ugent.be/en/courses/773/series/8693/activities/1555355466/

Supporting URLs for exercises that are not scoped in course would allow to make submissions for additional exercises (not included in course) using the pluging (by copy-pasting the exercise URL into the source code). Example:

https://dodona.ugent.be/en/activities/1555355466/

[pdawyndt]

BONUS: supporting course-scoped exercises that are not scoped in a series should work as well (haven't checked if this is already supported).