handle github api pagination when present (WIP)

[migueldiascosta]

alternative to #2553

needs vsc-base 2.8.4

[migueldiascosta]

I'm not particularly happy about the parsing of Links and traversing of pages here, it's certainly not pretty and I don't know if it is sufficiently robust. (It would be much cleaner using requests, but I suppose we are avoiding adding it as a dependency.) Open to suggestions/corrections.

[boegel]

@migueldiascosta This looks fairly OK to me, I don't see a better option w.r.t. grabbing the page count, since they do something very similar in https://developer.github.com/v3/guides/traversing-with-pagination/...

[wpoely86]

The code on which the rest code from vsc-base is based on has full pagination support since the beginning of this year: mozilla/agithub#49

So we might as well update the code in vsc-base and use that?