Add a schema checking concept to the web app.

This starts the concept of a schema checker, similar to
zerver/lib/validator.py on the server.  We can use this
to validate incoming data.  Our server should filter most
of our incoming data, but it's useful to have client-side
checking to defend against things like upgrade
regressions (i.e. what if we change the name of the field
on the server side without updating all client uses).

tools/test-js-with-node

@@ -54,6 +54,7 @@ enforce_fully_covered = {
     'static/js/reactions.js',
     'static/js/recent_senders.js',
     'static/js/rtl.js',
+    'static/js/schema.js',
     'static/js/scroll_util.js',
     'static/js/search_suggestion.js',
     # Removed because we're migrating code from uncovered other settings pages to here.