[RFC][Patch] Extend opendata plugin to support CKAN open data platforms

[Woazboat]

Extend the opendata plugin to be able to fetch resources from CKAN open data platforms.

​http://docs.ckan.org/en/2.9/user-guide.html#what-is-ckan

• New 'jackan' CKAN client plugin (​https://github.com/opendatatrentino/jackan)
• New CKAN data set handler that fetches resources via CKAN API
• Refactor opendata menu entry creation to allow modules to provide special menu item handling

Also adds a module for ​https://www.data.gv.at/ (WIP)

• Some sample CKAN data set handlers included for testing

TODO:

• Registered CKAN data set handlers currently send CKAN API requests on startup to fetch information about data set -> problematic, especially when number of handlers/queries increases
• Allow users to search and select datasets available on the open data platform via a user interface
• Eventually support ​https://data.europa.eu/en if possible (​https://data.europa.eu/api/hub/search/#tag/Ckan)