Newsletter template thumbnail redesign

Redesigned the template selection screen to
look like the WordPress theme selector.

The main changes relate to:
* style changes, to achieve the new look
* jsx templates, to work with new css changes
* template, removed the description field

[MAILPOET-1306]

lib/Config/PopulatorData/Templates/ScienceWeekly.php

@@ -15,7 +15,6 @@ class ScienceWeekly {
   function get() {
     return array(
       'name' => __("Science Weekly", 'mailpoet'),
-      'description' => __("The right chemistry to send your weekly posts.", 'mailpoet'),
       'categories' => json_encode(array('standard', 'sample')),
       'readonly' => 1,
       'thumbnail' => $this->getThumbnail(),