Set OmniAuth full_host parameter to ensure redirect URIs are correct

### What does this MR do?

This MR sets the OmniAuth `full_host` parameter to the configured GitLab URL to ensure the `redirect_uri` parameter is called with the right GitLab host.

### Why was this MR needed?

[OmniAuth attempts to grab the request URI](http://awesomeprogrammer.com/blog/2012/12/09/dealing-with-omniauth-redirect-uri-mismatch-invalid-port-number-gotcha/) and use that. If you set up a reverse proxy that terminates SSL at the Web server layer (e.g. https://gitlab.domain.com), omniauth will use the internal URL (e.g. http://my-host:8080) in its redirect URI unless all the Web server headers are properly set (e.g. `X-Forwarded-Port`, etc.). This is easy to forget or mess up, and it's better to ensure that OmniAuth has the right value from the start.

### What are the relevant issue numbers?

Closes #1967

See merge request !991