[v5]: Improve dashboard grid migration

[torkelo]

Migration code:
https://github.com/grafana/grafana/blob/develop/public/app/features/dashboard/dashboard_model.ts#L915

• When importing some dashboards the current migration has some issues. For example import this dashborad http://play.grafana.org/dashboard/db/alerting?orgId=1 and see that the first row & panel get the height wrong.

New grid system works like this, each panels is in a single array on the dashboard model. Each panel has a gridPos {x, y, w, h} these are in grid dimensions (width is 24 columns wide).

• Try the schema migration for many dashboards and identify issues and fix or improve the migration (might need to look at panel height as well). Ignore dashboards with panel/row repeats for now.

• Currently rows are ignored (or all rows are merged), need to detect if rows are used in the dashboard we import. And by used I mean if the row title is shown or if a row is collapsed. If either of those is true then add "row" panels to the dashboard.panels array.