Fix newsletter creation

some refactoring and two bugs fixed:
 - Tematicas dropdown wasn't getting the same data on new and on create
 - `tematica_id` field was been deliberately ignored, because it's not shown on the `edit`... although it's needed on the `create`
This doesn't seem very elegant.
Maybe the module should be completely renamed. Although it takes more time, it's not a fucking _chapuza_.