Rename debate-form HTML classes

So they follow the same convention used in proposals.

Note the styles are for elements which appear in the "new" view but not
in the "edit" view, so we only have to include them in one place.

app/assets/stylesheets/participation.scss

@@ -244,7 +244,7 @@
 // 02. New participation
 // ---------------------
 
-.debate-form,
+.debate-new,
 .proposal-new,
 .budget-investment-form,
 .document-form,
@@ -290,7 +290,7 @@
   }
 }
 
-.debate-form {
+.debate-new {
 
   .recommendations li::before {
     color: $debates;