Resolve "[Rails5] Fix `Route source can't be blank`"

What does this MR do?

In Rails 5.0 automatic inverse does not work for polymorphic relationships.
It was fixed in Rails 5.2.

This MR explicitly sets inverse_of: :source in the routable concern.

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

No.

Why was this MR needed?

Migration to Rails 5.0.

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 #46228 (closed)

#14286 (closed) and !12841 (closed)