Fixed bug in _AdvisingScreen.php, where the current student was lost when changing filters
in the Admin Toolbox

Fixed bug involving addition substitutions, where the max hours avail wasn't being calculated
correctly.

Added a safeguard when saving substitutions, so that you can never save for more hours than
the subbed course is worth.

Modified the db_error function in _DatabaseHandler.php, so we get a timestamp as well
as the backtrace emailed when an error occurs.

Added functions in db.inc for getting/setting individual user settings, which is required
for the changes to Student Search.

Updated student_search module, to have a major search tab, where advisors can see all students
in a major. Their selections are remembered in user settings. Also, added new permissions
for fine-level of control over who can see which subtabs.