Remove Risk-related models

* Models are already unused by the client side, so this just removes
  server-side pieces (tables, model definitions, tests, and endpoints)
Merge remote-tracking branch 'origin/develop' into feature/risk-assessments
Fix migration "down" revision
Enable extension migrations to use core `env.py` by default

* `env.py` in the extension will be used if available, otherwise use
  `ggrc/migrations/env.py`
Add `ggrc_risk_assessments` extension

* Add models, service endpoints, and migration for 5 initial models:
  Template, RiskAssessment, RiskAssessmentMapping, Thread, Vulnerability
* Add extension to default extensions for Behave tests and vagrant
  environment
Drop and re-create test database before running tests
Merge branch 'develop' into feature/risk-assessments

Conflicts:
	src/ggrc/views/__init__.py
Merge pull request #891 from reciprocity/feature/risk-assessments

Risk Assessment extension with DB and API