Add automatic search engine integration.

This adds the required metadata and files to support automatic search engine
additions to Chrome / Chromium.

r2/r2/public/static/opensearch.xml

@@ -0,0 +1,8 @@
+<?xml version="1.0"?>
+<OpenSearchDescription xmlns="http://a9.com/-/spec/opensearch/1.1/">
+  <ShortName>reddit search</ShortName>
+  <Description>Search for content on reddit!</Description>
+  <Url type="text/html"
+    method="get"
+    template="https://www.reddit.com/search?q={searchTerms}&amp;utm_source=opensearch"/>
+</OpenSearchDescription>

r2/r2/templates/base.html

@@ -35,6 +35,7 @@
     <meta name="description" content="${getattr(thing, 'short_description', None) or g.short_description}" />
     <meta name="referrer" content="${c.referrer_policy}">
     <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
+    <link type="application/opensearchdescription+xml" rel="search" href="/static/opensearch.xml"/>
     <link rel="canonical" href="${thing.canonical_link}" />
     %if hasattr(thing, 'mobile_link'):
         <link rel="alternate" media="only screen and (max-width: 640px)" href="${thing.mobile_link}" />