Migrate more secrets to application config file
Related to #1 (closed)
This MR migrates the secrets license_app_url and salesforce_host to the config/application.yml file. These items are more configuration than secrets. See #250 for more context. This follows the configuration pattern started in https://gitlab.com/gitlab-services/version-gitlab-com/-/merge_requests/726.
Once this MR is merged and deployed to production, we should be able to remove the following environment variables:
CUSTOMERS_ADMIN_URLLICENSE_APP_URLSALESFORCE_HOST