x/build/cmd/relui: re-deploying relui doesn't forget everything that happened

[toothrot]

cmd/relui needs a persistence layer. Let's say postgres! Or select a viable tool that works well in:

• development
• trybots
• production
• migrations

A local file on disk may not be enough.

[gopherbot]

Change https://golang.org/cl/340429 mentions this issue: cmd/relui: use go:embed for templates and content

[gopherbot]

Change https://golang.org/cl/340430 mentions this issue: cmd/relui, cmd/relworker: remove pubsub and datastore

[gopherbot]

Change https://golang.org/cl/347292 mentions this issue: cmd/relui: move implementation to internal/relui

[gopherbot]

Change https://golang.org/cl/347289 mentions this issue: cmd/relui: add Docker and Postgres configuration

[gopherbot]

Change https://golang.org/cl/347291 mentions this issue: cmd/relui: remove original implementation