[LinuxCNC/linuxcnc Issue#289] Transition our git to github

– [x] notify buildbot from github, not glo
– [x] IRC notifications from github on commit
– [x] e-mail commit notifications from github, not glo
– [x] wait for 2017-07-24
– [x] mark glo read-only (@cradek)
– [x] disable automatic mirror from glo to github (@cradek)
– [x] do one last manual mirror from glo to github (@cradek)
– [x] give github push permissions (@jepler)
– [x] merge #288 via github web ui (@jepler)
– [x] mailing list post on how to migrate existing clones (@jepler)