WIP: [Rails5] Add `ApplicationRecord`

What does this MR do?

1. Adds app/models/application_record.rb abstract file (this is a part of migration from official Rails guide).
2. Replaces ActiveRecord::Base with ApplicationRecord by rubocop --auto-correct command.
3. Adds a rubocop's cop with the autocorrection feature that checks for correct constant calls including inheritance.
4. Adds a spec for the new rubocop's cop.
5. Change rails' model generator to explicitly set ApplicationRecord as a model parent.

TODO

• Set --parent ApplicationRecord explicitly before rails' command gets unshifted in rails/commands.
  This step depends on !17828 (merged) as common code is used by both MRs.
• Fix typos in the spec it blocks (add trailing s).
• Update EE codebase when this MR is reviewed.

Why was this MR needed?

Migrating to rails 5.0

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?

#14286 (closed) and !12841 (closed)