Menu Set store, image pipeline and Site config

The store keeps a Menu Set's state in the directory layout (drafts/ versus
published/) and its metadata in a manifest, with publishing as a single
directory rename. Editing is allowed on Drafts only, so a published menu can
never change under a customer. Stages 2 and 3 of docs/plan.md, with tests.

.gitignore

@@ -0,0 +1,5 @@
+node_modules/
+.npm-cache/
+.tmp/
+*.log
+site.env