Update the system checks

• Remove arkindex.E006 (the Docker tasks image existence check), which does not make sense in production since the tasks image will usually be used by agents on other servers (and the backend doesn't need it)
  This adds an unnecessary step to the dev setup.
• Downgrade arkindex.E004 (local imageserver existence check) to a warning to allow access to the Django admin to configure said image server (although the devs now have a bootstrap script, production setups do not!)
• Include AWS_EXPORT_BUCKET in the S3 settings check
• Rename arkindex.W007 (the public_hostname warning) to arkindex.W008 because it conflicts with arkindex.W007 (the ponos private key warning)