Add UTC timestamps to the rss publication dates.

Without them, QuiteRSS was inferring the local time zone, which
changed with daylight savings time, which made all posts appear
as new even though the response from the server hadn't changed.

voussoir.net/writing/generate_site.py

@@ -465,7 +465,7 @@ def write_atom():
             <id>{{article.publication_id}}</id>
             <title>{{article.title|e}}</title>
             <link rel="alternate" href="https://voussoir.net/writing/{{article.web_path}}"/>
-            <updated>{{article.date}}</updated>
+            <updated>{{article.date}}T00:00:00Z</updated>
             <content type="html">
             <![CDATA[
             {{article.soup.article}}
@@ -493,7 +493,7 @@ def write_rss():
             <title>{{article.title|e}}</title>
             <guid isPermalink="false">{{article.publication_id}}</guid>
             <link>https://voussoir.net/writing/{{article.web_path}}</link>
-            <pubDate>{{article.date}}</pubDate>
+            <pubDate>{{article.date}}T00:00:00Z</pubDate>
             <description>
             <![CDATA[
             {{article.soup.article}}