feat(panettone): Use postgres as the storage backend

Switch from cl-prevalence to postgres (via postmodern) as the storage
backend for panettone. The first time the application starts up after
this commit, it will (idempotently) initialize the db schema and migrate
over all data from the prevalence snapshot to the database - the plan is
then to get rid of the prevalence classes and dependency once that's
deployed.

Change-Id: I4f35707efead67d8854f1c224ef67f8471620453
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1467
Tested-by: BuildkiteCI
Reviewed-by: tazjin <mail@tazj.in>
Reviewed-by: eta <eta@theta.eu.org>

web/panettone/src/util.lisp

@@ -0,0 +1,7 @@
+(in-package :panettone.util)
+
+(defun integer-env (var &key default)
+  (or
+   (when-let ((str (uiop:getenvp var)))
+     (try-parse-integer str))
+   default))