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).

client/components/main/layouts.js

@@ -1,13 +1,13 @@
-Meteor.subscribe('boards')
+Meteor.subscribe('boards');
 
-BlazeLayout.setRoot('body')
+BlazeLayout.setRoot('body');
 
-Template.userFormsLayout.onRendered(function() {
-  EscapeActions.executeAll()
-})
+Template.userFormsLayout.onRendered(() => {
+  EscapeActions.executeAll();
+});
 
 Template.defaultLayout.events({
   'click .js-close-modal': () => {
-    Modal.close()
-  }
-})
+    Modal.close();
+  },
+});