Unindexed GenericForeignKey relationships are very slow to delete

[YPCrumble]

First, thanks for maintaining this great library!

I'm getting very slow SQL times when deleting an action_object from my database if it triggers deletion of more than a few dozen notifications. Essentially, when a user deletes the action object, it requires deleting each of these hundred notification objects which takes a really long time. Per this Stack Overflow answer, it seems that an index on the appropriate GenericForeignKey field n the AbstractNotification model would solve the issue. Django also mentions it in the docs and refers to this open issue that the index might be created in the future: https://code.djangoproject.com/ticket/23435.

Would maintainers be open to adding this index by default, and potentially indexing each of the GenericForeignKey relations by default? If not, is there an easy way to do this simply in my application?

Thanks for considering it!

[YPCrumble]

Also, I'd be happy to create a PR if that's helpful!