Enable adding LHN sections from extensions

* Makes tooltips optional
* Makes actions customizable
* Adds "LHN.Sections" hook
Add "Risk" in LHN with "Risk Assessments" sub-section

* Adds `RiskAssessment` model client-side
* Adds custom templates and registers with "LHN.Sections" hook to show
  Risk section in LHN with "Risk Assessments" sub-section
Merge pull request #941 from reciprocity/feature/add-risk-assessment-ui

Add initial Risk Assessment UI