Generalize protected branch/tag dropdowns

What does this MR do?

CE backport: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/16420

• Generalize protected branch/tag dropdowns to create_item_dropdown
• Remove duplicate ee/ dropdown code

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

See "Protected Branches" and "Protected Tags" on <namespace>/<project>/settings/repository

Why was this MR needed?

This was generalized for usage in the new variable table design, see https://gitlab.com/gitlab-org/gitlab-ce/issues/39118#note_53615249

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
  • Not a user-facing change
• Tests added for this feature/bug
• Review
  • Has been reviewed by Frontend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan

What are the relevant issue numbers?