switch to gate for more generic policies

[WileESpaghetti]

Using gate might provide a way to do more generic policies to protect models. For example most of the models should only be readable/writeable by their creators, but with the current Policy classes that logic is duplicated for each model.

See:

• https://laravel.com/docs/9.x/authorization#creating-policies
• https://laravel.com/docs/9.x/authorization#gates
• https://stackoverflow.com/questions/61855105/one-policy-for-several-models