add support for gitlab authentication - #691

GITLAB_APP_ID=false
GITLAB_APP_SECRET=false
GITLAB_INSTANCE_URI=false

'gitlab' => [
        'client_id' => env('GITLAB_APP_ID'),
       'client_secret' => env('GITLAB_APP_SECRET'),
       'redirect' => env('APP_URL') . '/login/service/gitlab/callback',
       'instance_uri' => env('GITLAB_INSTANCE_URI'), // needs only for GitLab self hosted
       'name'          => 'Gitlab',
    ],