Kiwitcms-github-marketplace

- Fix a bug which allowed users to create multiple tenants
- Fix Sentry KIWI-TCMS-H2 <https://sentry.io/organizations/kiwitcms/issues/2584184445>
- Fix issues discovered by newest pylint
- Don't allow user to create multiple tenants if they refresh the page, e.g. after a 504 response. Instead redirect them to previously existing tenant
- Migrate from Travis CI to GitHub Actions
- Improvements of tests & CI

- 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