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/lib/utils.js

@@ -1,5 +1,5 @@
 allowIsBoardAdmin = function(userId, board) {
-  var admins = _.pluck(_.where(board.members, {isAdmin: true}), 'userId');
+  const admins = _.pluck(_.where(board.members, {isAdmin: true}), 'userId');
   return _.contains(admins, userId);
 };