Simplify privateRepoPublicSourcegraph check

Created by: lguychard

requestGraphQL() may be called recursively from performRequest() when retrying with a different URL.

When this is done from the background script (!isInPage), the second call would not be checked for privateRepoPublicSourcegraph(), which is an issue because that second call usually is to the public Sourcegraph.

The duplicate calls were in place because isPrivateRepository() relies on being executed in-page: see isPublicCodeHost, which would fail to properly detect a public code host if executed from the background page, and checks on the DOM in isPrivateRepository() itself.

This introduces the following changes:

• inject privateRepository as part of the request context in order to stop calling isPrivateRepository() from privateRepoPublicSourcegraph()
• remove duplicate privateRepoPublicSourcegraph() check: we no longer depend on this check being done in-page.

I added some tests for requestGraphQL(), made possible by optionally injecting ajaxRequest() to provide a stub for rxjs.ajax(). This is not the most elegant approach, but it at least allows to test the business logic in requestGraphQL(). See the new tests examplifying the fallback to public sourcegraph when the repo is not found on a public instance, for example.

The test errors if the repository is private and the user is logged out of his private instance would have failed prior to this fix.