improve volunteer import script to handle balancing assignments better

[struan]

There is always a lot of trial and error in getting this right so the script should do a better job of trying out an set of assignments and then backing it out. e.g. do it as a transaction and report on how that's gone and then can commit it if it all works out.

[zarino]

May be possible to fix #195 at the same time, but worth noting CEUK don’t actually use the volunteer upload page in #195, so that’s a lower priority.