Support Dataset.repository_link on all backends

[hagenw]

This is the second part of #4.

At the moment audbcards.Dataset.repository_link simply expects an Artifactory backend, e.g.

NOTE: the following code needs to executed using the add-test-db-fixture branch at the moment.

>>> import audbcards
>>> dataset = audbcards.Dataset('emodb', '1.3.0')
>>> dataset.repository_link
'`data-public <https://audeering.jfrog.io/artifactory/webapp/#/artifacts/browse/tree/General/data-public/emodb>`__'

One solution to make this independent of the used backend would be by introducing and if-clause. For example, we can return the above string if we detect the artifacory backend (as returned by audb.Repository.backend) and return just the name otherwise:

'data-public'

@ChristianGeng any thoughts on this?