Upgrade to Django 5.1

[ColonelPhantom]

Closes #3774

Summary

Upgrades to Django 5.1. Also uses the new {% querystring %} functionality. Additionally, it removes a use of get_storage_class because it was removed in this version. I looked at its implementation which just uses import_string so I decided to replace it with that.

How to test

1. Check that the site still runs.
2. Check that private media still loads.
3. Check that pagination still works correctly.

[ColonelPhantom]

One problem seems to be remaining:

FileField now raises a FieldError when saving a file without a name.

However, I'm not sure what name to give to these file fields, so I'm not sure how to work around this.

[ColonelPhantom]

Apparently we have a newsletter_filename function that overrides the filename field already...

Why the hell does Django mandate that we set it if it overrides it later anyway??

Anyway, I think using that function to get the name would be neater?

[ColonelPhantom]

Actually, it seems like upload_to is intended to override the given filename? So I think it is fine as is. Unfortunately I cannot review as I made the PR :(