Serve images from the CDN

Because it should be quicker than serving them from our app directly.

app/templates/views/service-settings/branding/email-options.html

@@ -26,7 +26,7 @@
         {% for option in form.options %}
           <div class="column-one-quarter">
             <label for="{{ option.id }}">
-              <img src="/static/images/branding-options/{{ option.data }}.png" alt="" class="bordered-image bottom-gutter-1-3">
+              <img src="{{ asset_path }}images/branding-options/{{ option.data }}.png" alt="" class="bordered-image bottom-gutter-1-3">
             </label>
             {{ radio(option) }}
           </div>

app/templates/views/signedout.html

@@ -54,7 +54,7 @@
         </p>
       </div>
       <div class="column-half">
-        <img src="/static/images/product/01-templates.svg" alt="">
+        <img src="{{ asset_path }}images/product/01-templates.svg" alt="">
       </div>
     </div>
   </div>
@@ -71,7 +71,7 @@
       </div>
       <div class="column-half">
         <img
-          src="/static/images/product/02-reporting.svg"
+          src="{{ asset_path }}images/product/02-reporting.svg"
           alt="A screenshot of GOV.UK Notify showing counts of emails and text messages sent"
         >
       </div>
@@ -88,7 +88,7 @@
           phone&nbsp;numbers and Notify sends the messages.
         </p>
         <img
-          src="/static/images/product/03-spreadsheet.svg"
+          src="{{ asset_path }}images/product/03-spreadsheet.svg"
           alt="A screenshot of a spreadsheet with columns for email address, name and colour"
         >
       </div>
@@ -98,7 +98,7 @@
           back office system.
         </p>
         <img
-          src="/static/images/product/04-api.svg"
+          src="{{ asset_path }}images/product/04-api.svg"
           alt="A screenshot of some computer code with a notify.send_email function"
         >
       </div>
@@ -180,7 +180,7 @@
       </div>
       <div class="column-half">
         <img
-          src="/static/images/product/team.jpg"
+          src="{{ asset_path }}images/product/team.jpg"
           alt="A photo of the GOV.UK Notify team gathered around a big screen. One team member is pointing at a graph on the screen."
         >
       </div>

app/templates/views/using-notify.html

@@ -37,7 +37,7 @@
     <h2 id="messagedeliveryandfailure" class="heading-medium">Sending messages</h2>
     <p>When you send a message, it moves through different states in Notify.</p>
     <img
-        src="/static/images/message-sending-flow.svg"
+        src="{{ asset_path }}images/message-sending-flow.svg"
         alt="A picture of the sending flow of messages in Notify, showing the three states of Sending, Delivered, And Failed. Also shows the next
              steps when messages fail, deleting data and trying a new channel for permanent failures, and trying again or trying a new channel for
              temporary failures"