This avoids loading the `OmniAuthCallbacksController` at boot time so
it doesn't mess up the `before_action`-chain

- Don't use `request.env['omniauth.params']` if it isn't present.

- Remove the `saml` section from the `gitlab.yml` test section. Some tests
  depend on this section not being initially present, so it can be overridden
  in the test. This MR doesn't add any tests for SAML, so we didn't really need
  this in the first place anyway.

- Clean up the test -> omniauth section of `gitlab.yml`

- Pass a `remember_me` query parameter along with the initial OAuth request, and
  pick this parameter up during the omniauth callback from
  request.env['omniauth.params']`.

- For 2FA-based login, copy the `remember_me` param from `omniauth.params` to
  `params`, which the 2FA process will pick up.

- For non-2FA-based login, simply call the `remember_me` devise method to set
  the session cookie.