Use common header app when viewing blobs

What does this MR do and why?

Ensures the common header app is used when viewing blobs

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Before	After

How to set up and validate locally

1. Go to a file in a repository
2. Add common_repository_blob_header_app=true param to the URL
3. Make sure you can use the blob header app and that everything works as expected

Related to #497051 (closed)