Slideshow serializer should validate template names

[holandes22]

Existance is really all we need, but we need to decide if this is really needed:

• The UI should on top display a list (with checkboxes) with the currently available templates.

adding a 'validate_templates' method to the serializer should do the trick.

[holandes22]

Added in 82494a65db821a187f8b4f4077932628d16bb5c9