Upgrade to python3 ckan2 11

[kovalch]

Make ckanext-switzerland python3 and CKAN2.11 compatible

Todo:

• Fix group snippets that used to rely on having access to c.group_dict
• Update controllers to blueprints:
  • perma
  • organization
  • group
• Update tests
  • test_blueprints.py
  • test_date_helpers.py
  • test_date_validators.py
  • test_decorators.py
  • test_format_utils.py
  • test_helpers_backend.py
  • test_helpers_frontend.py
  • test_helpers_localize.py
  • test_helpers_terms_of_use.py
  • test_logic.py
  • test_middleware.py
  • test_plugin_utils.py
  • test_search.py
  • test_subscription_emails.py
  • test_validators.py
• Require the updated-to-CKAN 2.11 version of ckanext-subscribe for the github tests, until we have merged that to main (Upgrade to python 3  ckanext-subscribe#9)
• Fix bug that stops us saving datasets to Solr because the datetimes are in the wrong format (isoformat but no Z at the end). This is presumably due to a change in date handling in CKAN core that we are skipping or overriding in our code.
• Adapt to new version of ckanext-hierarchy to make sure we always include child organisations when searching their parent organisation
• Set up database migrations for ogdch_subscribe plugin (see CKAN 2.11: Handling migrations for a plugin that extends another plugin ckan/ckan#9047)
• Figure out why search tests don't work on Github (are we setting up the Solr container wrong?)
• ...

Still to check for templates:

• macros
• scheming
• showcase
• source
• subscribe
• user
• xloader