Hide archived labels from auto completion

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What does this MR do and why?

This MR hides archived labels from auto completion. It's a follow-up to !196443 (merged) that added the ability to archive labels.

It's behind a feature flag called labels_archive.

🛠️ with ❤️ at Siemens

References

• Remove archived labels from quick actions and a... (#554226 - closed)

Screenshots or screen recordings

After enabling the feature flag and archiving the label named Brische:

Before	After

How to set up and validate locally

1. Enable feature flag with rails c:

    Feature.enable(:labels_archive)

2. Archive a label: Go to Any project > Manage > Labels and edit a label in the three dot dropdown. Check the Archived checkbox and save the label.
3. Now go to any issue in this project. In a comment, type /label ~ and wait for the auto-completion to show up. The archived label should now be hidden.
4. Auto-completions are cached for three minutes - you can reduce the cache to a second by editing app/controllers/concerns/autocomplete_sources/expires_in.rb and setting AUTOCOMPLETE_EXPIRES_IN = 1.second

MR acceptance checklist

MR Checklist ( @nwittstruck)

• 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 javascript style guides
• Conforms to the database guides

Related to #554226 (closed)