Add code navigation features like showing top-level files and directories for repo

[emagav]

I think code search tools are most helpful when you can also navigate the code with them, so you don't have to continually switch back and forth between your regular GH repo view and your GH code search view

Example features that would be helpful for this:

• On cs.github.com, if I select an organization, show a list of repos within that organization.
• If I select a repo, show a view of the top-level files and directories in this repo

This can pair really well with directory-scoped search too (I think this is already a FR): select a repo -> navigate down into a subdirectory by clicking through -> do a scoped search over that subdirectory