preview: add a ETag for browser caching of the previews

cmd/rednerd/cmd/serve.go

migration/scripts/0016_template_etag.up.sql

models/db_helpers.go

models/metadata_extra.go

models/template_db.go

restapi/embedded_spec.go

restapi/handlers/account-template-preview.go

restapi/handlers/document-file-response.go

...perations/template_management/account_template_preview.go

...template_management/account_template_preview_responses.go

swagger.yaml