Create customized feeds for the IATI Registry

[amercader]

We need a way to track changes on the registry (datasets edited or updated), globally and on a per country/publisher/etc. basis. RSS and Atom feeds are really popular, and after closing #191 and #1498 crating them from the search results should be fairly easy.

The following URLs are pretty self-explanatory:

http://localhost:5000/feed/registry.rss

http://localhost:5000/feed/country/AF.rss
http://localhost:5000/feed/publisher/worldbank.rss
http://localhost:5000/feed/organisation_type/10.rss

As we need to implement custom wrappers for countries, publishers, etc, we might as well offer a fully customizable feed, e.g.:

http://localhost:5000/feed/custom.rss?q=activity_count:[* TO 100]
http://localhost:5000/feed/custom.rss?publishertype=primary_source

Apart from the actual feeds, there will be a small amount of work at the template level to add the links to the suitable pages (and maybe a generic page showing all available feeds)

[amercader]

Added in 658f76e4a0 and deployed to the production site.