Sources list: display more source details

[StephenChan]

I thought it'd be good to branch this out from issue #206. That issue tossed out various ideas for making CoralNet's community of sources more browsable, including: additional columns in the source list/table, filtering that list/table, a 'featured sources' page, and making the list/table of sources a bit easier to find in the first place.

This issue can focus on a more narrow idea: that table/list of sources just has source names right now, but it should have more. What info should be added? What should be more prominent (in extra columns), or less prominent (in a tooltip), etc.

Possible info to display:

• Image count
• Date of last activity (perhaps last annotation will do)
• Fraction of images confirmed
• Annotation count
• Affiliation name
• Classifier accuracy
• Date source was created
• World region, based on lat/long
• Years covered in image metadata
• Labelset size
• Public/private (for site admins' view)

Some items may make more sense for site admins and less sense for public viewing.

Some items will be faster to retrieve from the database than others. Items should be cached and computed through periodic tasks as needed. If such caching is done, then at the end of the table, there should be a note saying something like "the image counts shown here are updated daily; visit the source to see the exact number."

• The source-list page already queries the DB for image counts of most sources. The reason is that this page also has the Google map of sources, which determines map-marker sizes based on image count. This query could use performance-profiling, though. The map is on the front page, so we want it to be pretty fast to load.