search contexts: Show dropdown only if user has added repositories

Created by: novoselrok

Only show search contexts dropdown if user has added repositories (determined by calling listUserRepositories).

The tricky part of this PR is showing the search contexts dropdown after user has just added new repositories (where previously he had none). This means that we have to send a signal all the way to the top to re-fetch the repositories and determine if we can show the dropdown. I briefly contemplated adding a full-page refresh after user finishes adding repositories, but that introduced additional weirdness.

I'm gladly taking suggestions on how to simplify this.