[feature request] List a number along the side of the grades summary table.

[alecjacobson]

When reviewing old grades, it's useful for me to be able to know if a student had, e.g., "the 9th highest score in the class" or "was in the top 20%". The current grades summary doesn't list the row numbers or tell me how big my selection is. So I either have to count the rows manually or paste the data into a different spreadsheet application. A simple extra column on the left with the row number would be enough.