Consolidate postgres-mixin

[Duologic]

The postgres-mixin has a place in upstream, should we consider removing it from grafana/jsonnet-libs before it diverges too much?

cc @kavirajk @jeschkies @tomwilkie

[tomwilkie]

Yes please!

[kavirajk]

Sure. Thanks @Duologic

[jeschkies]

Sure!

[stale]

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