All TOM Toolkit INSTALLED_APPs should inherit from this class
rather than the standard django.apps.AppConfig class that the
startapp management command uses by default. TOMToolkitAppConfig
defines (will define)  the instrospectable API of a TOM Toolkit app
and the methods tom_base will call for app-specific customizations.

This allows a TOMToolkit app's app config (i.e. TomTargetsConfig)
to extend (or not) the TOMToolkitAppConfig methods that tom_base
will call to introspect each TOMToolkit INSTALLED_APP.

These would be implemented in each INSTALLED_APP's subclass of
TOMToolkitAppConfig with the app-specific nav-bar content. I'm
not sure where these methods will be called from.