backport: new customization strategy for the provider message

We display a message for some providers like 'This is a demonstration provider.
Accounts may be deleted at any time.'

We used to do so by just customizing the entire home/content partial. This
had the downside that changes to this partial would not be reflected on the
customized providers.

Instead we now have a separate partial for that purpose.

app/views/home/_content.html.haml

@@ -5,6 +5,9 @@
 .row-fluid
   = home_page_buttons
 
+.row-fluid
+  = render 'home/provider_message'
+
 - if Rails.env == 'development'
   .row-fluid
     %hr

app/views/home/_provider_message.html.haml

+-# please overwrite me in your customization files at
+-# config/customization/views/home/_provider_message.html.haml