Make blob header bar sticky to the top of the page when user scrolls

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Context

See #585563 (comment 3080758879)

This is a nice-to-have feature enhancement. Make the blob header sticky so users always have access to buttons like the inline Blame button when they scroll through large files.

Design proposal

1. When users scroll, make the the blob header sticky to the top of the page so users can always access blob functions like the inline Blame button.
2. When the blob header is sticky to the top of the page, remove the border-radius.

Current	Proposed
Blob header disappears above the fold	Blob header is sticky Border radius removed when sticky