Show a message that map previews are deprecated

Although links will have been removed it is still possible to get to the map preview page if you have a direct url to it.
The code has been left in place to make it easier to turn map previews back on in the future.

app/views/map_previews/show.html.erb

@@ -29,6 +29,7 @@
   <%= link_to 'Back', :back, class: 'dgu-back-link govuk-link' %>
 <% end %>
 
+<div style="position: absolute; left:0; top: 0;color:yellow;z-index:100000000;width:100%;font-size:15pt;text-align:center;">Map previews on data.gov.uk have been retired. Please use an alternative method for viewing this map</div>
 <main role="main" id="content">
   <div class="grid-row inner">
     <div class="column-two-thirds ">