Added & cleaned up messages relating to a given user not existing

The following pages now have a <title> that can be set in localizations:

 * /user/USER_DOES_NOT_EXIST
 * /user/USER_DOES_NOT_EXIST/diary
 * /user/USER_DOES_NOT_EXIST/traces
 * /message/*/ID_DOES_NOT_EXIST

In addition I've cleaned up the i18n message keys of all the
''no_such_user.rhtml'' pages involved. They now all use
title/heading/body for the <title>, <h2> and <p> respectively. And the
message key {{user}} instead of {{name}}.