GitHub authentication option

[maael]

This PR adds github authentication as an authentication option on the site. It's implemented in a similar way to the Google authentication which was used as a basis.

Notes

• This PR is affected by issue https://github.com/sahat/megaboilerplate/issues/148.
• The default client id/secret will need to be updated, at the moment they've been left in to show it's working though.
• The login/signup buttons for the github authentication are not styled in any way and are just the default button style.

[sahat]

Thanks a lot for this PR @maael!