Activate and archive Imminence places for largest data sets

https://www.pivotaltracker.com/story/show/60332344
In order to keep Imminence's Place data to a manageable size for geoNear queries, archive everything but the latest 2 data set's places when activating the latest data set. Note this currently only applies for the two largest services, number-plate-supplier and motorcycle-approved-training-bodies.
This pull request contains the necessary migrations to clean the place_archives collection of superfluous data and to set the initial state for data sets.