updated DDREntity/Entity import in tasks.delete_file

Missed this one when I changed the imports in cf933fcb8a.
ask for confirmation before syncing collections

Sync is a long-running process that should not be interrupted,
so ask user to confirm before starting.
clarify object type in file-delete form.
added "cancel sync" button to sync-confirm page

Just in case the user forgets about the Back button...
made Sync button always available, even when in synced state
Merge branch 'presync-confirm'
refactor DDRLocalCollection.entities so it sorts correctly

- Used DDR.natural_sort instead of natural_order_string.
- Sorted the entity paths rather than the instaniated Entities.
more defensive coding of File.links methods

So we don't crash if there are no links, or vars used to make
paths used to find links are not as they should be.
updated Debian package version numbers in requirements file
new wsgi.py file and settings edit from latest Django version
rm settings file arg from supervisor gunicorn conf

Specifying the settings file caused Gunicorn to crash:
    TypeError "relative imports require the 'package' argument"
Merge branch 'master' into gitstatus-bkgnd

Conflicts:
	debian/conf/gunicorn_ddrlocal.conf
	debian/conf/settings.py
use latest gunicorn version and invocation