Putting this logic into the omniauth strategy is much cleaner because:
- we no longer need a Rails controller and the associated `custom_url` parameter
- we can re-use the `authn_request` instance which is automatically generated by the omniauth strategy, rather than re-implementing that logic
- the behavior is decided at runtime, rather than during initialization. This makes it testable, and is another step on the way to making the plugin multisite-compatible

This commit also introduces a spec for the feature.

The styling no works with Discourse's current login UI, so let's remove it. The OnceOff job was to migrate from an old data format more than 4 years ago. It's exceptionally unlikely that anyone is going to upgrade from such an old version. If they do, they could still extract the data manually and migrate it.

Our hosted sites running in stable branch have issues with `rexml` gem and returning "missing gem" error.

We started seeing [this error](https://github.com/onelogin/ruby-saml/issues/577) on some of our sites, which has been fixed on 1.12.1.

These were removed from core in https://github.com/discourse/discourse/commit/d2bceff133ac152678a1407d45fea260a0fe8536

This makes sure the saml_request can only insert strings into the HTML
form.

* Support for GlobalSetting.saml_base_url

* Push file with class method...

Co-authored-by: Richard <richard@discoursehosting.com>

- extract group_sync method
- run sync on new accounts
- only add user to groups that are both on the saml payload AND on the
  group list