The issue summary table sorting code is really old, referencing the now

non-existent Issue ID column, and making some assumptions about content

and attributes of the rows that no longer apply. Sorting the Last

Updated field didn't work correctly at all, given that the cells no

longer have a timestamp attribute.
This change rewrites the entirety of the sorting code, fixing timestamp

sorting, improving performance, and adding support for sorting in

descending order (by shift-clicking columns).
Sorting has also been improved to sort by issue ID when two values are

equal, keeping the order of issues consistent when two timestamps or two

authors are the same.

Tested sorting all columns in ascending and descending order, and

verified that equal values fell back on issue IDs (always in ascending

order).
Unit tests pass.