Settings page for Per Group SAML

What

Settings page for configuring Per-Group SAML, hidden behind enable_per_group_saml cookie.

Why

We're implementing Per-Group SAML to help customers manage group authorizations on GitLab.com. This first MR allows the integration to be configured and will be expanded on in a later MRs such as https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/4076.

Screenshots

Setup provider	Edit provider

Database Checklist

When adding migrations:

• Updated db/schema.rb
• Added a down method so the migration can be reverted
• Ordered columns based on the Ordering Table Columns guidelines
• Added foreign keys to any columns pointing to data in other tables
• Added indexes for fields that are used in statements such as WHERE, ORDER BY, GROUP BY, and JOINs
• Added the output of the migration(s) to the MR body

== 20180115013218 CreateSamlProviders: migrating ==============================
-- create_table(:saml_providers)
   -> 0.0140s
-- add_foreign_key(:saml_providers, :namespaces, {:column=>:group_id, :on_delete=>:cascade})
   -> 0.0038s
== 20180115013218 CreateSamlProviders: migrated (0.0179s) =====================

Acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Internationalization required/considered
  • TODO: Rebase on master and update translation files
• If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan
• End-to-end tests pass (package-qa manual pipeline job)

Related

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/4513