templates: add base URL for feed content (#2190)

Relative links in the entry content do not currently have a base URI, so
will be resolved relative to the feed URI:

Given an entry with the content:

    <a href="some-resource.bin">

And URIS of:

 * entry: https://example.org/blog/some-entry/
 * feed:  https://example.org/atom.xml

The link URI will end up as:

    https://example.org/some-resource.bin

rather than the URI that ends up resolved in the rendered page:

   https://example.org/blog/some-entry/some-resource.bin

The atom and RSS formats allow for an xml:base attribute (itself
specified in [1]) to provide a base URI of a subset of a document. This
change adds xml:base attributes to each entry, using the page permalink.

This gives us something equivalent to:

    <entry>
     <content xml:base="https://example.org/blog/some-entry/">
      <![CDATA[
       <a href="some-resource.bin">
      ]]>
     </content>
    </entry>

[1]: https://www.w3.org/TR/xmlbase/

Signed-off-by: Jeremy Kerr <jk@codeconstruct.com.au>

components/templates/src/builtins/atom.xml

@@ -40,7 +40,7 @@
         {% if page.summary %}
         <summary type="html">{{ page.summary }}</summary>
         {% else %}
-        <content type="html">{{ page.content }}</content>
+        <content type="html" xml:base="{{ page.permalink | escape_xml | safe }}">{{ page.content }}</content>
         {% endif %}
     </entry>
     {%- endfor %}

components/templates/src/builtins/rss.xml

@@ -33,7 +33,7 @@
           </author>
           <link>{{ page.permalink | escape_xml | safe }}</link>
           <guid>{{ page.permalink | escape_xml | safe }}</guid>
-          <description>{% if page.summary %}{{ page.summary }}{% else %}{{ page.content }}{% endif %}</description>
+          <description xml:base="{{ page.permalink | escape_xml | safe }}">{% if page.summary %}{{ page.summary }}{% else %}{{ page.content }}{% endif %}</description>
       </item>
       {%- endfor %}
     </channel>