Add translator documentation for a few files

* added a few XXX (that's StatusNet for FIXME, right?)
** proposing de-duplication of a message appearing ~50 times
** marking bad pagination implementation
* moved the TRANS: comments in action.php down to exactly before the line in which the message appears. Otherwise gettext does not put them in the pot file
* fixed a XXX in action.php removing double spaces

lib/htmloutputter.php

@@ -100,6 +100,7 @@ class HTMLOutputter extends XMLOutputter
             $type = common_negotiate_type($cp, $sp);
 
             if (!$type) {
+                // TRANS: Client exception 406
                 throw new ClientException(_('This page is not available in a '.
                                             'media type you accept'), 406);
             }