Open views naming the retired Snow style on Normal

createView threw on style 'snow', so an embed host or a
show_in_gods_eye_view call holding a view from before Snow was retired
was refused outright, camera and layers included. viewFromParams read
style=snow as the recipient's own style, while the app opened the same
link on Normal.

Both now read a retired style as Normal; truly unknown names still
throw. The retired list moves to a browser-free module shared by the
view API, the share codec and visual settings.

src/visualStylePolicy.js

@@ -0,0 +1,7 @@
+/**
+ * Browser-free style vocabulary shared by views, share links and the UI.
+ * Styles the app no longer offers, by the name share links used: a view,
+ * link, scene or shot that names one opens on Normal instead of keeping
+ * whatever style was already showing.
+ */
+export const RETIRED_STYLES = Object.freeze(['snow']);