Custom methods to `get_or_create`, `update_or_create`

[kevinetienne]

At the moment using django get_or_create or update_or_create doesn't work as some values might be encrypted/hashed. We can have a mixin which would override those methods from django.

[LilyAcorn]

We also need to include a create method. If an encrypted field is meant to be unique, we need to do a lookup to avoid accidentally creating duplicates.

[peterfarrell]

This enhancement request will be overcome by events (OBE) when the changes in the new_auto_decryption branch is merged into master.

[peterfarrell]

Fixed in v2.4.0