Modify AdminStatisticsController so that it doesn't hold all jobs in memory at once.

[brosenberg42]

This was causing OutOfMemoryErrors for users. This page is just calculating summary statistics, so it doesn't need to hold all of the jobs in memory at once. #1440 will need to completed first so there is a way to get the jobs lazily.

This will still be very slow when there are a lot of jobs, but at least it won't cause OutOfMemoryError. There might be a way to get the same information in SQL query, which would be significantly faster.