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`
yes, having module & class with the same name is a pain... but don't you think that renaming the class instead of the module could be less painful?