Add Web UI support for installations on user accounts

[carlosms]

Context in #456 (comment).

The web backend returns the list of organizations where the logged-in user is an administrator. But a GitHub App can also be installed on your user account instead of an organization.

This issue is to test if everything works with those installations, and enabling them once the web UI is more complete. This way we can develop the first version without premature feature creep.