Make new header app default

What does this MR do and why?

The new Vue app for header of Repository, Blob and Project Overview pages is temporarily hidden behind common_repository_blob_header_app=true URL param. This MR replace the url param with a derisk feature flag :blob_repository_vue_header_app.

Three wo additional changes:

• Add event tracking for ref selector (shared Repository header) #502383 (closed)
• Add logic to pass down new_workspace_path for the new workspace link in workspace dropdown, see !176182 (comment 2289738552)
• Fix a small UI issue in forked project overview page, found in a test session #512957 (comment 2300488827)

References

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• param was added in #497051 (closed)
• #509327 (closed)

MR acceptance checklist

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Screenshots or screen recordings

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	Before	After
Tree
Blob
Project overview (forked project)

How to set up and validate locally

1. Enable the feature flag

   Feature.enable(:blob_repository_vue_header_app)

1. See #512957 (closed) for breakdown of features to test

Related to #509327 (closed) #502383 (closed)