Kiwitcms-github-marketplace

- Don't fail when cancelling GitHub FREE subscriptions for senders which don't exist

- Fix traceback when trying to create tenant and user is not logged in

- Tested with Kiwi TCMS > 8.6
- Refactor deprecation warnings with Django 3.1
- Start using the new standard models.JSONField()
- Remove ``tcms_settings_dir/marketplace.py`` b/c ``settings.PUBLIC_VIEWS``
has been removed

- Require kiwitcms-tenants>=1.1
- Subscribe button is now a drop-down listing all platforms oferring a
Private Tenant subscription

- Bug fix: display form errors when creating new tenant
- Update template strings

- Turn into proper Kiwi TCMS plugin and install settings overrides under
``tcms_settings_dir/`` (compatible with Kiwi TCMS v8.2 or later):

- does not need ``MENU_ITEMS`` and ``PUBLIC_VIEWS`` override anymore
- does not need to load ``tcms_github_marketplace`` in ``INSTALLED_APPS``
manually
- Jump over ``tcms_tenants.views.NewTenantView`` b/c it requires
``tcms_tenants.add_tenant`` permission and here we don't need that
- Exclude public tenant from recurring purchase hooks
- Do not attempt delete for superuser cancelling their tenant purchases