Refactor some issuable JS code to move EE code out of CE files

What does this MR do?

Moves some EE code out of issuable CE code.

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/10020, https://gitlab.com/gitlab-org/gitlab-ee/issues/10089

Does this MR meet the acceptance criteria?

• 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