Separated private and public storage settings

Updated django settings to allow separating the storages used to store private and public keys:

JWT_PRIVATE_KEY_STORAGE

Storage class instance used to store JWT private signing key. Defaults to "ninja_simple_jwt.jwt.key_store.local_disk_key_storage".

JWT_PUBLIC_KEY_STORAGE

Storage class instance used to store JWT public verification key. Defaults to "ninja_simple_jwt.jwt.key_store.local_disk_key_storage".

0.3.0

Add settings to allow customization of web auth JWT cookie