Extend `public_config` implementation for extensions

* Basically, settings "exports" from extensions are now additive, and an
  extension-module-level method is available for providing
  permissions-dependent config.
* Also, add `ggrc_config` Mustache helper to make client-side config
  variables available in Mustache templates
Display "Superuser" in UI for "default admin" users

* Makes `BOOTSTRAP_ADMIN_USERS` config available to client-side, *only*
  to Admin users.
* Shows "Superuser" in top-right dropdown when applicable
* Shows "Superuser" for specified users on Admin page in Roles
Enable configuration of Risk Assessments UI URL

* `RISK_ASSESSMENT_URL` is now a config variable, and available on
  client-side for Admin users
* Use this new config variable in templates to link users to the correct
  external server