Fix comparison reporting

[#1]

Using the comparison reporting module is awkward due to the following UI bugs:

1. You can't edit a custom comparison item - once created you'd need to create it again, call it the same thing and then save over it

2. If you do edit a custom comparison as per above and it's already being used within a report you'll need to remove that comparison column and then add it back in - it doesn't update automatically

3. If you want to delete a custom comparison you can't unless you're prepared to delete all the custom comparisons you've added to that folder - this is perhaps the most annoying bug as one can very easily end up with a lot of custom comparisons that were used to test various things and now clutter up folders