Implement attachment application/pgp-signature

A detached signature doesn't make much sense in an email, but may happen
because of improper quoting. Or somebody might really want to send a
signature separate from the signed file.

mbox2web

@@ -101,6 +101,7 @@ def save_part(msg, disposition):
         "application/x-compressed-tar":            ".tar.gz",
         "application/vnd.oasis.opendocument.text": ".odt",
         "text/x-perl":                             ".pl",
+        "application/pgp-signature":               ".pgp",
     }[content_type]
 
     name = msg.get_param("name") or "(data)"

templates/attachment_application_pgp-signature.html

@@ -0,0 +1,9 @@
+<div class="partouter">
+    <div class="partheader">
+        application/pgp-signature
+    </div>
+    <div class="partinner">
+        <a href="{{url}}">{{name}}</a>
+    </div>
+</div>
+