Since that works fine, most of this docstring is misleading. Since the first part is at best out of date (contains no reference to the fact there are two classes defined in the module), I really cannot be bothered fixing it - yes it's technically a PEP257 violation, but I'd rather no docstring than a misleading docstring, and any IDE should be capable of giving you a far better indication what is going on in the module.
Fix many PEP8 line-length violations. Because onereallyreallyreallylonglineofcodethatseemslikeitgoesonforever is kind of hard to read :)
We should inherit from admin.ModelAdmin here - the code depends on `self.model` and `self.admin_site` which make it unlikely to be a truly generic mixin anyway. Bonus: Fixes PyCharm warnings.
I wonder is these actually do anything, e.g. is the default behaviour to add another popup necessitating the need for a `dismissAddAnotherPopup`?