Archived Labels: Add unarchive button to edit label view

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What does this MR do and why?

Archived Labels: Add unarchive button to edit label view

This MR adds an unarchive button in an alert to the edit label view.

🛠️ with ❤️ at Siemens

References

• Split active and archived labels (#554227)
• Archived labels (#4233)

Screenshots or screen recordings

How to set up and validate locally

1. Enable feature flag via rails c:

Feature.enable(:labels_archive)

2. Archive a label: Go to Any project > Manage > Labels and archive a label in the three dot dropdown. It should now show up in the Archived tab.
3. Go to the Edit view of the archived label. Now, you should see the box on the top and should be able to unarchive it.

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 #4233