Omniauth redirect_uri wrong when using https

The "redirect_uri" from omniauth is always http, even when the gitlab-time-tracking app is served as https.
For this reason, gitlab always response with this error:

```
The redirect uri included is not valid.
```

One possible solution is to set the variable "OmniAuth.config.full_host" to the correct URI. Maybe we can configure the host url through an env variable? Or auto-detect it correctly?
