`name` is nil... so this logic was not working at all

All data will be automatically migrated from `oauth2_user_infos` to the `user_associated_accounts` table

GlobalSettings configured via environment variables will continue to take precendence, so this change is backwards compatible.

- `.presence` is added to string values, since 'unset' site settings are never `nil`
- saml_force_domains is split on `|` and `,` because site settings expect a `|`, while the old global setting expected a `,`
- Specs are migrated to use SiteSettings, because GlobalSettings do not override SiteSettings in test mode

This centralises our logic for accessing settings. This particular commit should be a no-op. I intend to followup with the new site setting definitions in a future commit.

This means that the name/title can be set per-site and per-locale. This change is backwards-compatible - any existing sites which have configured the GlobalSettings will use those cluster-wide.

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>