Enforce a consistent ES6 coding style

Replace the old (and broken) jshint + jscsrc by eslint and configure
it to support some of the ES6 features.

The command `eslint` currently has one error which is a bug that was
discovered by its static analysis and should be fixed (usage of a
dead object).

server/publications/unsavedEdits.js

@@ -1,5 +1,5 @@
 Meteor.publish('unsaved-edits', function() {
   return UnsavedEditCollection.find({
-    userId: this.userId
+    userId: this.userId,
   });
 });