Add debates translation interface

Also fix broken spec after removing translatable attributes from
strong_parameters definition. Now we need to send these attributes
as nested translations attributes.

Use activerecord.yml title attribute label so form helper could load it
from default location.

config/locales/en/activerecord.yml

@@ -181,7 +181,7 @@ en:
         author: "Author"
         description: "Opinion"
         terms_of_service: "Terms of service"
-        title: "Title"
+        title: "Debate title"
       proposal:
         author: "Author"
         title: "Title"

config/locales/es/activerecord.yml

@@ -183,7 +183,7 @@ es:
         author: "Autor"
         description: "Opinión"
         terms_of_service: "Términos de servicio"
-        title: "Título"
+        title: "Título del debate"
       proposal:
         author: "Autor"
         title: "Título"