migrate to passport-github2

[outofambit]

GitHub recently deprecated API authentication through query parameters. #1618 removed all the calls we were making directly from the web editor, but it looks like this might still be a problem from these lines, where we use passport-github. passport-github hasn’t migrated from the deprecated GitHub API auth strategy and probably won’t since it seems to be unmaintained now (jaredhanson/passport-github#75).

But! someone has forked that repo and fixed this issue here: http://www.passportjs.org/packages/passport-github2/. Moving to passport-github2 should be enough to fix this. (it appears to be a drop in replacement for passport-github, so it might be as simple as changing the dependencies, but i haven't looked too closely.)

[welcome]

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