Move header search translations to shared constants.js

What does this MR do and why?

We're splitting off New Global Search Modal MR into a couple of MRs. That MR has already UX, backend, and a first round of frontend approved though it was suggested by frontend maintainer that it would be easier to review it in smaller parts. So this is the first part where we're moving the translations from header search to the new shared location to be reused in the following MRs.

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The paths to the final state

👣 Current MR

👣 Second MR

👣 Final MR (Link to be provided)

How to validate

No visual or behavioral changes are expected - the header search should work as before.