Add ability to save attachments using a `FileField`

[joshuadavidthomas]

https://github.com/treyhunner/django-email-log/blob/461f2ced7bd447ac2c22868fa331520d7dc3798a/email_log/models.py#L43

[joshuadavidthomas]

Yes, I think this would be a good improvement. Keep the default of saving the attachments in the database as it currently is, but add the option to store it in a FileField in a separate model to leverage different storage.

[joshuadavidthomas]

My initial thought on a seamless way to do this is look if settings.STORAGES["email_relay"] is set. If it is, use an Attachment model with a file field with that storage backend. Otherwise, use the database as it currently works.

[joshuadavidthomas]

Ran into a bug where I had tried to send a poorly encoded CSV attachment and it totally ground the relay to a halt. This should be prioritized.