Improve performance for feedback views

[membralala]

Short description

This PR should improve performance for the admin and region feedback views as it reduces the amount of database queries.

Proposed changes

• Add some cached_properties in the feedback models
• Add an external dependency django-polymorphic (https://django-polymorphic.readthedocs.io/en/stable/)

[svenseeberg]

@membralala can you please elaborate why the polymorphic module improves performance? The reasons for making the change is not really clear to me. I expect that you did not find this by chance?

[timobrembeck]

Can you please elaborate why the polymorphic module improves performance? The reasons for making the change is not really clear to me. I expect that you did not find this by chance?

@svenseeberg This is the explanation: https://django-polymorphic.readthedocs.io/en/stable/performance.html#performance
The get_real_instance() mentioned there is roughly equivalent to our Feedback.submodel_instance().

[timobrembeck]

Thanks a lot 😍

Since django-polymorphic improves the performance pretty significantly and makes it possible to remove some of our "magic" submodel-instance code (which ultimately improves maintainability of our code base), I think it's justified to add another dependency here. Also, the other changes look very clean and straight-forward to me, good job!