Add Github Authentication

[eMerzh]

As discussed on Gitter,

we should probably add GitHub Auth for the weblate.

https://docs.weblate.org/en/latest/admin/auth.html?#github-authentication

For that we probably need to:

• create a GitHub App with callback URL looking like (If I'm correct) https://translate.photoprism.app/accounts/complete/github/
• Edit environment file, uncomment GitHub related keys (WEBLATE_SOCIAL_AUTH_GITHUB_KEY, WEBLATE_SOCIAL_AUTH_GITHUB_SECRET)
• party 🎉 ?