Extract ee model concerns

What does this MR do?

This moves ee specific model concerns in preparation for single codebase.

Changelog entry added, if necessary
Documentation created/updated via this MR
Documentation reviewed by technical writer or follow-up review issue created
Tests added for this feature/bug
Tested in all supported browsers
Conforms to the code review guidelines
Conforms to the merge request performance guidelines
Conforms to the style guides
Conforms to the database guides
Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
EE specific content should be in the top level /ee folder
For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?
Security reports checked/validated by reviewer

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