Use forked github-markup gem

Merged Gabriel Mazetto requested to merge bug/rst-disable-patch into master

To use the forked version, we must undo some patching we did before.

I'm preserving one patch that changes the interpreter from python2 to python3 so that we don't change the required libraries for current source installs. This will let us release the change in a patch release instead of waiting for 8.13.

Before accepting this MR we must:

Related issue: gitlab-org/gitlab-ce#21696

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