Save and display summary stats from source-wide exports

[StephenChan]

Prerequisite: #373 (summary stats)

This applies to both percent coverage and calcification rate stats.

• When a stats export is done over all images in the source, save the results to a database field. (Perhaps a field in the Source database table.)
• Display the latest source-wide export statistics in a box on the source main page. Include the export date and number of images used in the stats computation, to give an idea of how up-to-date it is.
• If no source-wide exports were done yet, we still display a small box on the source main page, indicating that export results would go there. This should help the discoverability of the export features.
• In the export form, indicate whether or not the export stats will be saved, based on whether the current selection includes all images in the source.

The source main page display requires saving stats from past exports, because it takes a while to compute stats over many images, and we don't want to slow down the source main page significantly.

Since there are endless ways to filter a source to a subset of its images, I thought the simplest thing to do (at least for a start) would be to limit this save feature to the 'all images' case.

[StephenChan]

Decided not to do this task right away, since I wasn't entirely sure about the current performance of image covers / calcification rates exports, and wasn't sure if it'd be wise to add more incentive to do full-source exports yet. The performance situation might be better than I thought, but I'll err on the safe side for the moment.