[Dashboard] Fix study progression performance

[driusan]

For long running studies the "study progression" widget on the
dashboard was doing an excessive number of queries on the database
(one per site per month of the study having been running) which
was causing the dashboard to take an excessive amount of time to
load.

This replaces it with a single query using a GROUP BY, and then
massages the data in PHP to the same format.

To test this compare the dashboard/ajax/get_scan_line_data.php results
before and after this change. The JSON returned should be identical, but the
performance should be faster (if there's a large enough amount of data
that the before was noticeably slow to load.).

[driusan]

(Note: get_recruitment_line_data probably should get a similar treatment, but the query on that one isn't quite as expensive so it takes seconds instead of minutes to run on my sandbox.)