Do not show the "View file" button if repo is private but instance is public

Created by: lguychard

Fixes #3065 (closed)

I originally started this branch to work on #3072 (closed), but realised that the solution to that was more complex than anticipated, because since extensions must work on private code, we can't just nuke hover/codeintel functionality if the repo doesn't exist on the Sourcegraph instance.

In any case, the changes I started allowed me to fix #3065 (closed), and also to deduplicate a lot of code in the browser extension:

• It was previously up to CodeViewSpec.resolveFileInfo() to call ensureRevisionsAreCloned(), making sure the code view's revision(s) existed on the Sourcegraph instance. This was error-prone - in several instances, ensureRevisionsAreCloned() was not called.
• Some instances of CodeViewSpec.resolveFileInfo() would also fetch the file contents, but the downstream code would assume that the file contents needed to be fetched anyway.
• I made sure that ensureRevisionsAreCloned() and fetchFileContents() were only called once per code view, in the codeViews observable.

With ensureRevisionsAreCloned() now being reliably called for every code view, I added FileInfo.privateRepoPublicSourcegraph, and used it to hide the "View File" button when the repo doesn't exist on the public sourcegraph instance. With a private Sourcegraph instance, ensureRevisionsAreCloned() will throw if the revision doesn't exist, and the code view event won't be emitted.