Bugfix:  Saving changes to Decisions via admin screen raises exception
that an object with that primary key already exists. This is caused
by DecisionAdmin's save_model() method passing request.user to
obj.save(), where Decision.save() passes all parameters to
model.Model.save(), which expects its first argument to be force_insert,
so db engine tries to do an insert rather than an update, which explains
the surprising error message (see publicweb/admin.py,
publicweb/models.py and django/db/models/base.py).
DecisionAdmin.save_model was originally added as part of a fix for csv
export 2 years ago where at the time Decision.save() was expecting
author as the first parameter but not passing it to models.Model.save().
Decision.save() not longer expects author as first parameter, and a load
of code that used to be there 2 years ago is now gone, so it seems safe
to just delete DecisionAdmin's save_model method. Can now save
Decisions via admin screen.
https://aptivate.kanbantool.com/boards/5986-econsensus#tasks-1393515
Update Decision.editor for edits via admin screens too.
Drop all decision watchers on a Decision when it gets moved to a new
Organization.
Tidied up test_changing_decisions_org_drops_decisions_watchers.
Made test more thorough (and caught a bug - author of an old feedback
getting notified):
- make one of each type of change (decision update, new feedback,
  feedback update, new comment) before *and* after moving the decision to the
  new organization, checking that no-one from the old org gets notified
upon changing the decision in the new org.
- use a different user for each change so that at least one email
  notification should go out for each change (we often don't notify the
original author or the editor)
Ensure we delete the feedback's watchers for all of the Decision's
feedbacks. Also, added test highlighting the next level that needs
fixing - watchers for django comments!
Ensure that we delete the watchers for all comments, for all feedbacks
under the decision.
Tidied up test_changing_decisions_org_drops_decisions_watchers()
Tidy up, added comments.
When Decision's organization is changed, add all members of new org as
watchers of Decision, its Feedbacks and their Comments. New Decisions
always get all current org members as watchers, but only the author is
usually a watcher of Feedbacks and Comments. However, in this special
case, these may well end up with no watchers if the original author is
not a member of the new org, so get everyone to watch them instead.

When a brand new Decision has its organization changed via admin screens,
ensure that notifications sent out (to new org members) are the usual
'your Decision has changed' type, not the status change type, by setting Decision's
last_status to its status in DecisionAdmin.save_model(), as is done for user screen edits.

Refactored tests so that change_organization_via_admin_screens() is now
in open_consent_test_case.py, so that view_tests can call it too, rather
than duplicate code.

https://aptivate.kanbantool.com/boards/5986-econsensus#tasks-1393515
Remove import no longer needed.
Removed imports that are no longer needed.
Merge pull request #112 from jogwen/allow_decision_moves2

Phase 1 on allowing Decisions to be moved from one Organization to another (via admin screens only atm) https://aptivate.kanbantool.com/boards/5986-econsensus#tasks-1393515