Fix content warning issues over https

When served with HTTP/2 and TLS, project pages show several mixed
content errors:

* the imported Google fonts
* the AJAX call to spring.io to get the current project information
* the newsletter iframe

This commit changes URLs to use https or protocol-relative URLs when
possible. This also deletes the now defunct newsletter integration.

_config.yml

@@ -36,6 +36,6 @@ custom_gradle_template: false
 
 ### The following properties are constant for most projects
 
-main_site_url: http://spring.io
-projects_site_url: http://projects.spring.io
-questions_url: http://spring.io/questions
+main_site_url: https://spring.io
+projects_site_url: https://projects.spring.io
+questions_url: https://spring.io/questions

_includes/page.html

@@ -4,7 +4,7 @@
     <link rel="icon" type="image/png" href="{{site.baseurl}}/favicon.png"/>
 
     <link rel="stylesheet" type="text/css" href="{{site.baseurl}}/bootstrap/css/bootstrap.min.css" />
-    <link href='http://fonts.googleapis.com/css?family=Varela+Round|Montserrat:400,700' rel='stylesheet' type='text/css'>
+    <link href='//fonts.googleapis.com/css?family=Varela+Round|Montserrat:400,700' rel='stylesheet' type='text/css'>
     <link rel="stylesheet" type="text/css" href="{{site.baseurl}}/font-awesome/css/font-awesome.min.css" />
     <link rel="stylesheet" type="text/css" href="{{site.baseurl}}/bootstrap/css/bootstrap-select.css" />
 
@@ -114,15 +114,9 @@
                         </div>
                     </div>
                     &copy; <script type="text/javascript"> var d = new Date();
-                    document.write(d.getFullYear());</script> <a href="http://www.pivotal.io">Pivotal Software</a>, Inc. All Rights Reserved.
-                    <a href="http://www.pivotal.io/terms-of-use">Terms of Use</a> and
-                    <a href="http://www.pivotal.io/privacy-policy">Privacy</a>
-                </div>
-                <div class="span4 footer-newsletter--wrapper pull-right">
-                    <div class="footer-newsletter--container">
-                        <label>Subscribe to our newsletter</label>
-                        <iframe frameborder="0" height="42px" scrolling="no" src="http://play.gopivotal.com/OSS_Website_Spring_SpringNewsletterSubscriptionEmailOnly.html" style="border:none" width="324px"></iframe>
-                    </div>
+                    document.write(d.getFullYear());</script> <a href="https://www.pivotal.io">Pivotal Software</a>, Inc. All Rights Reserved.
+                    <a href="https://www.pivotal.io/terms-of-use">Terms of Use</a> and
+                    <a href="https://www.pivotal.io/privacy-policy">Privacy</a>
                 </div>
             </div>
         </div>