Mention security as a key feature on the web site "Features" page

Spinoff from http://search-lucene.com/m/l6pAi1ghxBm21t0a2

On the Solr web site http://lucene.staging.apache.org/solr/features.html we list key features.
Now with 5.3 out the door, I think one of those icons should be about security. Proposed text:

Title: Security built right in
Subtitle: Secure Solr with Authentication, Role based Authorization and SSL. Pluggable, of course!

We could design a new icon for security, but instead, here's a suggestion to repurpose the existing ones:

• Remove the "External configuration" part - not very impressive anymore
• Instead use the "External config" icon for the "Schema-less feature".
• Use the “schemaless" icon (with a key) as the new security icon

See how it looks here: http://www.cominvent.com/solr/Apache%20Solr%20-%20Features.html