All users with attempts in any state are now listed in the overview report.

Fixes #7 - "Main overview report doesn't list users with open attempts and no completed attempts"

With this fix in place, teachers can delete the single open attempt for a student
if an error occurred and the student didn't complete the attempt.
The main overview report now lists 'n/a' as a score if no valid score was recorded.
Reports now use the HTML character code for +/- rather than multiple characters.