* Alerting NG: prototype v2 (WIP)

* Separate eval package

* Modify eval alert definition endpoint

* Disable migration if ngalert is not enabled

* Remove premature test

* Fix lint issues

* Delete obsolete struct

* Apply suggestions from code review

* Update pkg/services/ngalert/ngalert.go

Co-authored-by: Kyle Brandt <kyle@grafana.com>

* Add API endpoint for listing alert definitions

* Introduce index for alert_definition table

* make ds object for expression to avoid panic

* wrap error

* Update pkg/services/ngalert/eval/eval.go

* Swith to backend.DataQuery

* Export TransformWrapper callback

* Fix lint issues

* Update pkg/services/ngalert/ngalert.go

Co-authored-by: Kyle Brandt <kyle@grafana.com>

* Validate alert definitions before storing them

* Introduce AlertQuery

* Add test

* Add QueryType in AlertQuery

* Accept only float64 (seconds) durations

* Apply suggestions from code review

* Get rid of bus

* Do not export symbols

* Fix failing test

* Fix failure due to service initialization order

Introduce MediumHigh service priority and assign it to backendplugin
service

* Fix test

* Apply suggestions from code review

* Fix renamed reference

Co-authored-by: Kyle Brandt <kyle@grafana.com>