Make the form GDPR ready (#329)

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**Make the docsearch form GDPR compliant**

Needed to be 100% GDPR compliant. 

**Result**
Check the code

docs/source/assets/javascripts/main.js

@@ -44,6 +44,7 @@ window.addEventListener('load', function() {
       var $email = $(this).find('input[name="email"]');
       var $url = $(this).find('input[name="url"]');
       var $owner = $(this).find('input[name="owner"]');
+      var $marketingOptin = $(this).find('input[name="marketing_optin"]');
       var error = false;
       $(this).find('.has-errors').removeClass('has-errors');
       if (!$email.val()) {
@@ -74,7 +75,9 @@ window.addEventListener('load', function() {
           'https://go.pardot.com/l/139121/2016-08-05/ldp67?email=' +
           encodeURIComponent($email.val()) +
           '&website=' +
-          encodeURIComponent($url.val())
+          encodeURIComponent($url.val()) +
+          '&marketing_optin=' +
+          encodeURIComponent($gdprEmail.val())
         );
 
         $('.join-form-fill').hide();

docs/source/index.html.haml

@@ -24,7 +24,7 @@ layout: default
 
 #section_why.page-section.pos-rel
   .spacer40
-  .m800.m-l-r-auto.pos-rel.z-10
+  .m800.m-l-r-auto.pos-rel.z-5
     %h2.page-section-title.upper-case.text-demi Why DocSearch?
     .spacer32
     %p.color-east-bay.text-regular We're kind of scratching our own itch here. As developers, we spend a lot of time reading documentation, and it isn’t always easy to find the information we need.

docs/source/partials/join-form.haml

@@ -20,7 +20,16 @@
     %label
       %input{type: 'checkbox', name: 'owner'}
       %span I'm the <u>owner</u> of that website
-  .spacer32
+  .row
+    .form-group.pos-rel.p-small
+      %div
+        %input.fill-titan.pos-rel.p-r-small#marketing_optin{name: "marketing_optin", type: "checkbox", value: 'true'}
+        %label.text-sm{for: "accept_email", style: "display:inline;margin-left: 8px;letter-spacing: unset;font-weight: 500;text-transform: initial"} Yes, I'd like to receive more information on Algolia products, events and promotions via email
+      %div
+        %p 
+          Refer to 
+          %a{href: "https://www.algolia.com/policies/terms", class:"clickable"} Algolia's Privacy Policy 
+          for more information on how we use and protect your data
   .text-center
     %button.btn.btn-static-primary.w100p{type: 'submit'}
       Join the program