database: views for prebuilds

[dannykopping]

prebuilds view

Encapsulate the logic of how prebuilds are identified

• a prebuild is identified as a workspaces row being owned by the prebuilds user (database: migration for prebuild user #298)
• every prebuild will only have one workspace_builds entry with a start transition, and its initiator_id will be set to the prebuilds user

prebuilds_reconciliation view

Encapsulates the logic of how prebuilds must be scheduled

• desired count: instances specified in the coder_workspace_preset resource per active template version
• actual count: number of running prebuilds per active template version
• extraneous count: number of running prebuilds per inactive template version

[dannykopping]

Status update:

I opted to move the prebuilds_reconciliation view into a query rather than a view; we'll likely not need to reuse this query in other queries.

This work is now ~complete, but may need to be revisited later.