Kiwitcms-github-marketplace

- 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

- Replace `ugettext_lazy` with `gettext_lazy` for Django 3.0

- Compatible with PyGithub v1.45+ which will be used in the upcoming Kiwi TCMS v7.3

- utils.verify_signature() moved to tcms.utils.github as of Kiwi TCMS v7.2
- flake8 & pylint fixes

Fix a bug in reading pricing info when renewing subscriptions via FastSpring