Search: Add link to RSS feed in Browse

[m-i-l]

User feedback has suggested that some people primarily use searchmysite.net to find sites to add to their RSS feeds.

Now I'm indexing content type as per #63 , I've found that many/most sites have an RSS feed indexed, e.g. search for content_type:*/xml NOT url:*sitemap.xml. It should therefore be possible to auto-populate a value for the RSS feed for many/most sites.

It may therefore be useful to add a link to a site's RSS feed (if it has one) on the Browse page.

The Browse page uses an is_home:true filter, so it would make sense to add the rss_feed attribute only where is_home:true, similar to the api_enabled attribute.

As with other schema changes, will need to wait a few weeks for enough sites to be reindexed to populate the new data, before the front end can be updated to show it.

[m-i-l]

This is now auto-populating the rss_feed field for a lot of sites. I have however found at least one site which has a clear link on the home page to its RSS feed, but where the RSS feed link isn't being indexed at all (let alone being identified for the rss_feed). The link is specified in link rel="alternate" type="application/rss+xml" href="..." and does not return any Content-Type http header. This makes me wonder if there is still value in having a column in the database for users to specify their RSS feed - search will use that if there is one, and only try to auto-populate if there isn't. Note that there wouldn't be any attempt to copy the auto-populated RSS feed value back from the search engine to the database (data only goes in one direction - from the config database to the search engine, not vice versa).

[m-i-l]

Note Solr field has been renamed from rss_feed to web_feed, and there are two fields in the database: web_feed_auto_discovered and web_feed_user_entered. See #54 .