Improved support for accessing models when using multiple databases

[Spikiii]

This was an issue that kept popping up in a work project that utilizes django-polymorphic. Specifically, the issue was that django-polymorphic didn't respect multiple databases for deletion, causing it to repeatedly error out if the object didn't exist in the default database, but did exist in the target_db.

I've verified that this change passes the tests, as outlined in the community contribution guidelines.

[j-antunes]

Hi @Spikiii - The project has recently moved under a new team. If possible could you rebase this PR against master?

Also, could you provide a unit test for you changes? Thanks!