Subscriptions for CKAN

Features:
- users can subscribe to a search
- search will recognize whether or not the search is already subscribed to
- users get more activities (activities of the datasets that matches the subscribed searches)
- different types of subscriptions can be added via plugin interface (extension can be seen here: https://github.com/srkunze/ckanext-semantic)


Questions to be resolved:
- when is a user allowed to follow a search? limit of search results? minimum number of search criteria?
- how can be asserted that a subscription name is unique for each user