Support in-context and in-line editing for Blobs
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### Proposal
Make it easier for users to quickly edit a blob without losing the context they're in when seeing the blob page.
See PoC MRs in:
* https://gitlab.com/gitlab-org/gitlab/-/merge_requests/116057
* https://gitlab.com/gitlab-org/gitlab/-/merge_requests/116050
* https://gitlab.com/gitlab-org/gitlab/-/merge_requests/116055
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issue