rollout feature flag for `saas_posthog_backend`

rollout feature flag for saas_posthog_backend .

Feature flag added in MR: !628 (merged)

How to test it

This is a feature designed only for JH saas.

Before using, need to configure environment variables:

• POSTHOG_API_KEY
• POSTHOG_API_HOST

Use JH SaaS like ordinary users, such as visiting a public project page, and then observing data changes in the PostHog Live Events tag.

@QiangGuo