feat(19.3-04): purge the host site's page cache after FAQ saves and deletes

- FormAfterCreate/FormAfterUpdate run the locale check, then register a
  purge of kind faq through cooler.PurgeSiteCacheAfterCommit
- new FormAfterDelete purges after the delete commits
- purgeSiteCache helper takes the save transaction from cabana.TxFromContext
  and the purger from cooler.SiteCacheFromApp (site_cache.* keys)
- TestFaqAdminPurgesSiteCache on Postgres

controllers/site_cache.go

@@ -0,0 +1,32 @@
+package controllers
+
+import (
+	"context"
+
+	"git.golem15.com/golem15/summercms/modules/backpack"
+	"git.golem15.com/golem15/summercms/modules/cabana"
+	"git.golem15.com/golem15/summercms/modules/cooler"
+)
+
+// purgeSiteCache registers one site-cache purge of kind and slug, sent by
+// the framework's purger (site_cache.purge_url and purge_token) after the
+// save transaction of ctx commits. Without a purge configured, or outside a
+// save transaction, it does nothing. A purge never fails the save.
+func purgeSiteCache(ctx context.Context, app func() *backpack.App, kind, slug string) {
+	if app == nil {
+		return
+	}
+	a := app()
+	if a == nil {
+		return
+	}
+	tx, ok := cabana.TxFromContext(ctx)
+	if !ok {
+		return
+	}
+	purger := cooler.SiteCacheFromApp(a)
+	if purger == nil {
+		return
+	}
+	cooler.PurgeSiteCacheAfterCommit(ctx, tx, purger, kind, slug)
+}