Add relation to slides using this template

This will be useful when we allow updating the template svg so we can regenerate the slides using this template.
Move template getter and setter next to each other
Read simple slide template settings from current_event

This is the first stepping-stone towards configuring the simple editor from Events. Currently the settings-hash is just static, but Soon(TM)
Fix create test

We changed the parameter fields a little so the test post data needs to also be updated.