Fix OmniAuth OAuth2Generic strategy not loading

In https://github.com/rails/rails/commit/83b767ce, Rails 5.1 removed support for using a String to specify a middleware. When the strategy_class argument is passed from the GitLab YAML config to Devise, Devise passes the string value straight through to Rails, and GitLab would crash with a NoMethodError inside ActionDispatch::MiddlewareStack.

To make this OmniAuth strategy work again, we normalize the arguments by converting the strategy_class value into an actual Class.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/62216