Dedupe identity verification alert strings into ci constants

What does this MR do and why?

The pipeline editor tabs and the pipeline header both render the account verification alert with the same description and button text. This moves those two shared strings into app/assets/javascripts/ci/constants.js so the copy lives in one place. The per-surface titles differ and stay inline at each call site.

Closes #629176

Follow-up to !254829 (merged)

Before / After

Before: The identity-verification alert's description and button text were duplicated inline in both the pipeline header and the pipeline editor tabs, so a copy change had to be made in two places.

After: Those two shared strings live in IDENTITY_VERIFICATION_ALERT_I18N in ci/constants.js, and both surfaces reference it. The rendered copy is unchanged; the per-surface titles stay inline.

How to set up and validate locally

1. Run the affected component specs:

   yarn jest spec/frontend/ci/pipeline_editor/components/pipeline_editor_tabs_spec.js spec/frontend/ci/pipeline_details/header/pipeline_header_spec.js

2. Or in the UI (as a user required to verify identity): confirm the alert copy on the pipeline header and the pipeline editor tabs is unchanged.

MR acceptance checklist

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Related to #629176