In translated forms, pass default_locale to fields and widgets (bug 894563)

https://bugzilla.mozilla.org/show_bug.cgi?id=894563

Before this patch, when creating input/textarea widgets for empty values, we used the current language. At creation time the user can't see or edit fields in this locale if it's not the same as the app's, so this created an empty Translation for those fields.

With this, we now use the app default_locale, avoiding creation of empty Translations when submitting, since the user will be editing fields in that locale.