Reimplement task checking validity of links

[vronk]

Reimplementation of previous checkurl task,
there was also option to "ignore" the validation results (stored as separate field) in add course page

[patrickakk]

For an example of the importance of this task, the link below shows an 404 (not found), when clicking on "Source URL" for a course which has been updated last week:

https://dhcr.clarin-dariah.eu/courses/view/367

Assigned to November milestone, since September is more than full.