🐛 Subscribe overlay fixes

refs. https://github.com/TryGhost/Casper/issues/630

- added a dedicated close button element for the subscribe overlay which sits on top of all elements to ensure the modal is always closable
- fixed alignment and spacing of "Subscribe" button for mobile sizes

default.hbs

@@ -48,7 +48,8 @@
     </div>
 
     <div id="subscribe" class="subscribe-overlay">
-        <a class="subscribe-close" href="#"></a>
+        <a class="subscribe-close-overlay" href="#"></a>
+        <a class="subscribe-close-button" href="#"></a>
         <div class="subscribe-overlay-content">
             {{#if @site.logo}}
                 <img class="subscribe-overlay-logo" src="{{@site.logo}}" alt="{{@site.title}}" />