Centralize all mutations at the model level

This commit uses a new package that I need to document. It tries to
solve the long-standing debate in the Meteor community about
allow/deny rules versus methods (RPC).

This approach gives us both the centralized security rules of
allow/deny and the white-list of allowed mutations similarly to Meteor
methods. The idea to have static mutation descriptions is also
inspired by Facebook's Relay/GraphQL.

This will allow the development of a REST API using the high-level
methods instead of the MongoDB queries to do the mapping between the
HTTP requests and our collections.

client/lib/unsavedEdits.js

@@ -65,7 +65,7 @@ UnsavedEdits = {
 };
 
 Blaze.registerHelper('getUnsavedValue', (fieldName, docId, defaultTo) => {
-  // Workaround some blaze feature that ass a list of keywords arguments as the
+  // Workaround some blaze feature that pass a list of keywords arguments as the
   // last parameter (even if the caller didn't specify any).
   if (!_.isString(defaultTo)) {
     defaultTo = '';