Results page: Optionally show subtotals for each question group

[christianp]

Suggested by Ben Brawn.

[haleematallat]

Hi, I would like to work on this @christianp

[christianp]

@haleematallat thanks for offering to help. Please look at our guidance on contributing to Numbas. Make a pull request if you make some changes for me to look at, or let me know if you need any help.