Django-oscar

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- Add new multi-encryptor class that combines multiple other encryptors together. This allows key rotation and graceful migration between different encryption methods.

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- Handle pending application responses separately from denied responses. They now throw different API exceptions with different error messages and error codes.
- Add some basic dashboard view tests.

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- Add foreign key from TransferMetadata to APICredentials used to make the transfer.

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- Fix bug which prevented adding new plan groups via the dashboard.
- Adds unit tests for financing plan and financing plan group dashboard forms.

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- Save last 4 digits of resulting account number to credit application models.
- Add ``TransferMetadata.purge_encrypted_account_number`` method.
- Handle ValidationError when submitting a transaction to prevent 500 errors in checkout.
- Fix 500 error in Credit App API when SOAP API returned a validation issue.
- Fix install documentation regarding API credentials.

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- Fix bug when migrating account numbers to new encrypted fields.