Add tests for the encryption
Add tests for the encryption

Check that:

- Password is not stored in plain text in Redis;
- The token returned has the expected format;
- The key returned is indeed the decryption key;
- API backwards compatibility is maintained: passwords stored in plain text
  can be retrieved via the original URL token.

`test_returned_token_format` superseeds `test_set_password`, which
was only validating the key length.

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