Use subscription in the block name

[MAILPOET-1798]

assets/js/src/post_editor_block/blocks.jsx

@@ -1 +1 @@
-import './form_block.jsx';
+import './subscription_form/form_block.jsx';

assets/js/src/post_editor_block/subscription_form/form_block.jsx

@@ -4,7 +4,7 @@ import Edit from './edit.jsx';
 const wp = window.wp;
 const { registerBlockType } = wp.blocks;
 
-registerBlockType('mailpoet/form-block', {
+registerBlockType('mailpoet/subscription-form-block', {
   title: window.locale.subscriptionForm,
   icon: Icon,
   category: 'widgets',