fix: accept github urls longer than organization/repo segments

[pierrecamilleri]

• fixes Cannot access URLs #1607

It looks like reading user (or organization) and repository names for the Github portal from the URL are faulty, as it was not expecting more than two URL segments.

I relaxed this constraint, and parsing user (or org) and repository names now work even if the URL continues.

I performed the manual test :

fricitonless validate https://github.com/DCMLab/dcml_corpora/releases/download/v2.1/dcml_corpora.datapackage.json

that seems to work.

Additionnal thoughts :

• all tests related to github portal are actually skipped ! pytest.mark.skip doc
• I added some tests to confirm the above behavior.
• Documentation code for github portal was not working because it was pointing to a non-existant repo. I changed it a little with another repo.

[pierrecamilleri]

Sorry, I was on the wrong track: I hadn't quite understood how the Github portal worked.

Validation seemed to work because the /DCMLab/dcml_corpora repo also contains files to be validated, but this is not the expected behavior for the url provided.