Add feature flag for planner feature #1107

Priority

None

Feature Description

Add feature-flag that allows users to opt-into this feature before its generally available

Implementation

Ensure compatibly with:

• Posthog | best practices, reducing costs
• local development with minimal setup
• Hosted Compass

Out-of-Scope
These can be done in separate issues.

• Serving feature-flags behind a reverse proxy, like Posthog suggests.
• Identifying users using posthog's identify() (docs).

Use Case

This'll help users experience the planning feature early and allow us to gather feedback without breaking the core features of the app

Additional Context

bootstrapping feature flags in Posthog: That is, assigning default values for the feature flag