Move the token creation/renewal code into the NordigenClient and remove
external access. The NordigenClient now also takes the DB client in the
constructor.
While creating the unit test, noticed that the nordigen mocks for
downloaded transactions could match the simple transactions mock, thus
helping in the online download command.
Also created the nordigen.NordigenClient mock, with the used methods
mocked and raising a NotImplement when a new endpoint is requested.
CSV is not a good format, since it does not have a good support for
complex types, e.g. lists.
It served when the exported format was only a linear transaction, but it
is not worth the extra work around it anymore.
