Add Favicon to OPDS Feed

<icon> is a standard of the OPDS spec, and this commit adds the favicon to the feed. Certain OPDS readers, e.g. FBReader on Windows, respect this tag and will show the same icon as the main page.

cps/templates/index.xml

@@ -1,5 +1,6 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <feed xmlns="http://www.w3.org/2005/Atom">
+  <icon>/static/favicon.ico</icon>
   <id>urn:uuid:2853dacf-ed79-42f5-8e8a-a7bb3d1ae6a2</id>
   <updated>{{ current_time }}</updated>
   <link rel="self" href="{{url_for('opds.feed_index')}}" type="application/atom+xml;profile=opds-catalog;kind=navigation"/>