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/config/reactiveTabs.js

@@ -1,5 +1,4 @@
 // XXX Since Blaze doesn't have a clean high component API, component API are
 // also tweaky to use. I guess React would be a solution.
-ReactiveTabs.createInterface({
-  template: 'basicTabs'
-});
+const template = 'basicTabs';
+ReactiveTabs.createInterface({ template });