improve translatability of 'archivebot-older-than'

by adding a 'duration' parameter

Per mailing list:
http://comments.gmane.org/gmane.comp.python.pywikipediabot.general/14160

Corresponding change in the bot script:
Ie4699550def701a728abaca3e62287e1c36ce3b0

Change-Id: I507b0b38a87fe8e97b850a71d9c1764ec16fbeb1