No code intelligence on Github blame views

[lguychard]

• visit https://github.com/sourcegraph/sourcegraph/blame/master/shared/src/api/extension/types/selection.ts
• witness there is no code intelligence
• witness error in the console:

Uncaught Error: Unable to determine the file path of the current file because the current URL (window.location https://github.com/sourcegraph/sourcegraph/blame/master/shared/src/api/extension/types/selection.ts) does not have a file path.
    at Object.resolveFileInfo (file_info.ts:71)
    at MergeMapSubscriber.project (code_intelligence.tsx:522)
    at MergeMapSubscriber._tryNext (mergeMap.ts:141)
    at MergeMapSubscriber._next (mergeMap.ts:132)
    at MergeMapSubscriber.Subscriber.next (Subscriber.ts:100)
    at MergeMapSubscriber.notifyNext (mergeMap.ts:170)
    at InnerSubscriber._next (InnerSubscriber.ts:18)
    at InnerSubscriber.Subscriber.next (Subscriber.ts:100)
    at MergeMapSubscriber.notifyNext (mergeMap.ts:170)
    at InnerSubscriber._next (InnerSubscriber.ts:18)

See also errors on Sentry, like https://sentry.io/organizations/sourcegraph/issues/976935753

(I don't think we ever did anything to support code intelligence on blame views, I just noticed this in the Sentry exceptions)

[github-actions[bot]]

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