Remove Spinach

What does this MR do?

Removes the Spinach gem.

Todo

• rebase on master when !18868 (merged) is merged
• rebase on master when !18867 (merged) is merged
• rebase on master when !18794 (merged) is merged
• clean up docs
• clean up gitlab-ci.yml
• clean up Gemfile both for rails4 and rails5
• clean up rake tasks
• clean up and remove /features directory when all specs are migrated (see above)
• create an EE port

Are there points in the code the reviewer needs to double check?

Double check I cleaned up everything that should be done so.

Why was this MR needed?

Spinach removal campaign.

Screenshots (if relevant)

No.

Does this MR meet the 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
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes #23036 (closed)