GitHub Two Factor Authentication

Summary:
I have added support for GitHub's two factor authentication when authorizing accounts, I have not yet looked at account revocation.

This has only been tested with my personal GitHub account.

Reviewers: mwolff, kfunk, ial0

Reviewed By: kfunk

Subscribers: mwolff, kfunk, kdevelop-devel

Differential Revision: https://phabricator.kde.org/D2621