[search] Leader board notification

This is an idea to implement leader boards in the platform re-using the search app.

1. Implement a new attribute to the Search model called experiments, pointing to the current set of experiments which are the result of a search. N.B.: This field is not used to display the search results, which still relies on the code available at the app. It is only used for the Leader Board feature. This feature is switchable with a simple checkbox and, Search.experiments is not visible otherwise to the user.
2. For every Search with experiments set, perform the following via a cronjob (once every 4 hours?):
   1. Re-run the search
   2. Check if the new set of experiments is the same as before
   3. If it changed, report this via e-mail (and on the actstream?) of the owner, indicating the old and new lists (relative differences may be displayed). The e-mail contains a link to the original search
   4. If (3) was fullfilled, for each user that has an experiment on the search AND access to the search, report the relative displacement of the user experiment's position.