Allow all reviewers to be visible in sidebar "Reviewers" edit dropdown

What does this MR do and why?

For #501135 (closed)
Makes every selected reviewer a visible reviewer when rendering the high-level "edit" reviewer dropdown from the merge request sidebar.

This does not affect the approval rule drawer reviewer dropdowns.

References

• Filtering out ineligible reviewers: #481798 (closed), !169669 (merged)

MR acceptance checklist

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Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Enable the reviewer_assign_drawer feature flag ==> rails c; Feature.enable( :reviewer_assign_drawer )
2. Assign a user as a reviewer of an MR
3. Open the reviewer edit dropdown (on the primary sidebar, not in the drawer)
   • Previously: No reviewers show
   • Now: All selected reviewers show