Add other Risk Assessment-related objects to LHN

* Adds Threat, Vulnerability, and Template to left-hand nav/search (again, only for Admin users).
* Clean up extension mechanisms for services, views, and configuration from extensions
* Bugfix for `mapping_counts`
* Fixups for attribute declarations in `RiskAssessment` and `Template` objects.
* Includes changes to display "Superuser" in the UI when applicable.
* Enable configuration of external Risk Assessments URL.