Cleverer 'update' mechanism

[Daniel Holbach]

The LoCo Directory gets more and more functions and more and more external data find its way into the LD. We don't need to udpate all the data in every cron run.

 - update-countries: once a day (even less would be fine)
 - update-languages: once a day (even less would be fine)
 - lpupdate:
   + get list of teams (every 10-15 minutes?)
   + get list of admins/owners (every 10-15 minutes?)
   + get statistics (bug 605651) (every 2 hours?)
   + get all team members (so we can appoint loco contacts) (every 30 minutes?)
 - etc. - there will be even more stuff in the future.

To make sure we don't have too many concurrent cronjob going on, I suggest we kick off "./manage.py update" every 10 minutes. It itself should keep track of which job needs to be run in which intervals and let the jobs leave stamp files somewhere, so we can check when it was last run.