Save to storage

[ysegorov]

Hi, Ilya!
We were in need to keep pretty large files in cache (10Mb or larger) and actually it works with memcached and it's great 👍
At the same time somehow it depends on memcached version and stability of keeping file content in cache was an issue

So, I've decided to keep data to be put in cache at minimum and save file content to django's default storage
And I've added some helpers to allow AdminResubmitMixin to be overridden with custom cache instance if needed

I'm not sure if all of this should go to upstream - just tell me and if yes I'd be happy with pr
My modifications are here

Thank you for the library :)