update volunteer import script to be more flexible

This adds the following features:
* a dry run mode to allow seeing what the end result will be without
  updating the database
* a debug more which has diagnotstic information about how many
  assignments are being made to each user
* an option to read assignment counts from the import file
* an option to read preferred assignment council types (country or LBO
  only at the moment) from the import file

It also will not assign Audit users to councils they marked as part of
the First Mark.

add dry_run option to volunteer import script

Update the script to run all changes inside a transaction and if the
dry_run option is passed then rollback all changes.
This lets you see any issues with running out of assignments without
having made any changes to the database.

enable reading assignment counts from the import data

allow volunteer import data to have council restrictions

so you can limit people to e.g. Welsh councils.

allow superusers and ceuk staff to have existing assignments

Mostly to account for them having multuple rows in the same spreadsheet

for non First Mark assignments do not assign to council already assigned

This is mostly for the Audit stage where users should not be auditing
their own work

use response type in volunteer import where required

add more bad section names to section name map

add debug option so can output more verbose info

change restricted councils to be councils to select from first

Author: struan