Allow / fix admin search

The admin search function was broken for Datasets and Datafiles (they used a search non-existent field 'experiment__id').  I changed it so that Experiment, Datafile, Dataset, Replica and Location can all be searched by >>their<< id.  

(More useful / intuitive than searching by experiment id, IMO.  And besides I'm not sure that search by experiment id could work anymore given there is now a many-to-many relationship between Experiments and Datasets.)