Rename Topics -> Domains

[DavidLeoni]

DKAN uses the so-called Topics to organize datasets

• Topics are only a feature of DKAN, they don't appear as fields in CKAN.
• By default they are not-hierarchical

For now we can adopt them and rename into hierarchical domains. Possible problems:

• what about multilinguism? We would need to provide a new translation for each language
• Topics are not meant for being a hierarchy, see #21

[DavidLeoni]

Preserved changes to DKAN Topics Features by using Feature Override, see #23