gitlab: use /oauth/userinfo and respect GITLAB_URL

[gartnera]

Gitlab supports the standard /oauth/userinfo oauth/odic endpoint so we should use that instead of gitlab specific API calls.

Also allow setting the gitlab url via environment variable. Still defaults to https://gitlab.com.

Also set DefaultScopes correctly. Users should be instructed to grant their gitlab app to grant the following scopes: email openid

Closes #118

[coveralls]

Coverage increased (+0.3%) to 91.682% when pulling d9e8166 on gartnera:master into b759a6a on tarent:master.

[matfiz]

@smancke it would be so great if you could find a moment to review and merge this to master in the nearest future.

[xadereq]

@smancke bumping request above ^

[gamalan]

definitely need this

[gartnera]

Test failure is unrelated.

amd64 build of caddy is located here: https://gitlab.com/gartnera/caddy-gitlab-login/-/jobs

[g-w]

Hello @gartnera,
could you please rebase your PR to the current master. After that I will have a look into your changes and merge them.

[xadereq]

Hello @gartnera,
could you please rebase your PR to the current master. After that I will have a look into your changes and merge them.

@g-w Looks like it's rebased, could you take a look at it? It's been almost a year..

[xadereq]

Well, it looks like COVID-19 will eventually finish our living in misery faster than you making decision about merging this PR. Bumping...

@g-w