Fix for required tags duplication & improvements to tag renaming

[Oaphi]

closes #939

Given that the fix is extremely minor, I've thrown in a couple of improvements to tag renaming, specifically, aligned the JSON response structure with the other methods (part of #1754) + the failure message is now server-side.

Note to reviewers: new locale string files are only picked up by Rails servers only after reload

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.33%. Comparing base (9bf4235) to head (1c4e61b).
⚠️ Report is 6 commits behind head on develop.

Additional details and impacted files

Components	Coverage Δ
controllers	70.33% <100.00%> (+0.02%)	⬆️
helpers	81.92% <ø> (ø)
jobs	60.00% <ø> (ø)
models	86.29% <ø> (ø)
tasks	61.11% <ø> (ø)

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[cellio]

LGTM. Tags no longer duplicate (for either edits or suggested edits). It looks like the other changes are internal, or at least I couldn't figure out a user-visible change that I could test manually.

[Oaphi]

It looks like the other changes are internal

Yeah, no user-visible changes have been made to tag renaming, just a bit of cleanup in preparation for #1754