Display sign in buttons in code view toolbar

Created by: felixfbecker

The sign in button currently shipped is only shown on the repo-level view on Sourcegraph button, which is not shown on Bitbucket PR views.

This shows them in code views instead, if there is a code view on the page. This connects the call to action closer to the context where the user gets the benefit.

Closes #9276 (closed)

cc @christinaforney