* backend/dashboard snapshots: refactor leftover models and mocks
* Move all dashboard snapshot-related models into the dashboardsnapshotservice package
* Remove leftover dashboard-related mocks from the mockstore
* backend/sqlstore split: move dashboard snapshot funcs to dashboardsnapshotservice
This commit moves the dashboard snapshot related sql functions in the dashboardsnapshots service. I split the dashboards package up so the interfaces live in dashboarsnapshots and the store and service implementations are in their own packages. This took some minor refactoring, but none of the actual underlying code has changed, just where it lives.
