Rename proposal-form HTML class to proposal-new

So it's consistent with the proposal-edit class we use in the edit
action.

app/assets/stylesheets/admin.scss

@@ -315,7 +315,7 @@ $table-header:   #ecf1f6;
   }
 
   .admin-content {
-    .proposal-form {
+    .proposal-new {
       padding-top: 0;
     }
   }

app/assets/stylesheets/participation.scss

@@ -245,7 +245,7 @@
 // ---------------------
 
 .debate-form,
-.proposal-form,
+.proposal-new,
 .budget-investment-form,
 .document-form,
 .topic-new,
@@ -297,7 +297,7 @@
   }
 }
 
-.proposal-form,
+.proposal-new,
 .document-form,
 .topic-form,
 .topic-new {
@@ -308,7 +308,7 @@
 }
 
 .budget-investment-new,
-.proposal-form,
+.proposal-new,
 .proposal-edit,
 .polls-form,
 .poll-question-form,