Confirm dialog when changing total size of pools

[norbye]

When pools are changed in size, what concerns the organizers is the total amount of participants - and currently they have to manually sum up the size of all the pools to see that..

As it's troublesome to remove people that will already have been bumped after they realize their mistake it would be nice to have;

1. A visual field in the edit sidebar that displays the sum of available spaces, and
2. A confirm dialog that also displays the total count (only when the size differs from the last edit)