feat(alerting): lots of progress on notifications, refactored them out to their own package, restored webhook notitication and added slack notification

Added grafana-cli  command. Added public documentation for specifying plugin version when installing. (#5665)

Conflicts:
	package.json

feat(templating): fixed issue with template variables set to reload on dashboard load being issued multiple times, fixes #5637

The key returned by `GET /api/dashboards/db/:slug` is "dashboard" (not "model").