Improve condition slider percentage logic

[tomjemmett]

The sum of the conditions sliders should not exceed 100%: currently this is enforced by setting the max-value of each slider independently to enforce this limit.

Instead, we should set the max-value's to 100% and use the following algorithm:

1. if the percent sum exceeds 100% find the minimum percentage of the other sliders (the ones that haven't been updated)
2. calculate the amount we need to reduce the sum by to reach 100%, and then equally divide this value by the number of sliders we are working with
3. take the minimum value of 1 and 2, then reduce all of the sliders by this amount
4. if the sum still exceeds 100%, remove the sliders which are now at 0% and repeat