Revamp Git Blame display so it leaves the code cleaner

Created by: Joelkw

We have both internal (see attached concept from @rrhyne) and external feedback that our git blame display makes reading the code more difficult. We should consider changing how we display it.

Considerations:

• This might involve creating another contribution point(s) to the extensions API, which is not a minor lift.

Created by: github-actions[bot]

Heads up @joelkw @felixfbecker @jeanduplessis - the "team/web" label was applied to this issue.

Created by: AlicjaSuska

This also aligns with the feedback I've gathered from uses when testing the action items bar.

Created by: github-actions[bot]

Heads up @joelkw @felixfbecker - the "team/extensibility" label was applied to this issue.

Created by: Joelkw

Additional concept/feedback:

A prospect on a demo today asked if we had anything similar to this particular blame history view in GH: https://github.blog/2017-01-18-navigate-file-history-faster-with-improved-blame-view/. He liked the inline blame, he liked the file history, but he wanted the history for the line, essentially. (At least that was my understanding of the thing he sent and the way he described it.) Am I missing a way to nab that, or is that specific sort of view just not an option?