Convert atom feeds to use feedgenerator library.

Issue is that Werkzeug > 1.0.0 has removed werkzeug.contrib.atom.AtomFeed,
making it difficult to use a distribution-packaged version of werkzeug. To solve
this, I've replaced use of werkzeug.contrib.atom.AtomFeed with
feedgenerator.Atom1Feed.

After the change, the only major difference between the feeds before and after is
that they use <summary> instead of <content>. Minor differences include no longer
adding 'type="text/html"' on some <link> elements and no "xml:base" attribute on
<entry> elements. I don't think these differences will have any noticable
effect.

Tested on Liferea feed reader.

docs/source/siteadmin/relnotes.rst

@@ -20,6 +20,15 @@ Release Notes
 This chapter has important information about our current and previous releases.
 
 
+0.12.0 (work in progress)
+=========================
+
+**Improvements:**
+
+- Switch Atom feeds from deprecated werkzeug.contrib.atom to feedgenerator,
+  upgrade werkzeug (Ben Sturmfels)
+
+
 0.11.0
 ======