Reimplement login/password authentication via OAuth2

[slonopotamus]

Even with gitlab-10.2 it is still possible to authenticate using login/password pair via OAuth2: https://docs.gitlab.com/ce/api/oauth2.html#resource-owner-password-credentials

For example, that's how we did it in git-as-svn: slonopotamus/git-as-svn@b434b39#diff-3af4173981772a146cfacf15b38b9531

[gmessner]

OAuth2 login support has been added to release 4.7.7. GitLabApi.oauth2Login() methods have been added, and GitLabApi.login() methods automatically try the OAuth2 login if the session login fails with a 404 error.