Separate selectors used on blame page

What does this MR do and why?

Describe in detail what your merge request does and why.

Separates elements and methods used on the blame page into a new page object as it still uses the old file view while repository files have been refactored to use the new blob view with refactor_blob_viewer feature flag (that is now released).

Follow-up to !91757 (comment 1019769883)

MR acceptance checklist

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