No entrypoint for managing repos if the user hasn't added a code host

Created by: arussellsaw

Now that we have the 'public repos' box, we want users to be able to add those repos without connecting a code host, however right now there's no way to access the manage repos page without adding a code host. we should also probably make sure the manage repos page understands the current state when loaded without a code host, as it will show options that don't make any sense at the moment, eg 'selected repos'. we should probably get rid of the entire top box.

this is a blocker for #19139 (closed)