Integrate PostHog custom events #1159

In much the same way that AntAlmanac Scheduler maintains a list of custom events and sends those to PostHog, Planner should also use custom events so we can improve our analytics for specific features.

We should include a minimal set of events (more can be integrated after this change is merged):

• Create custom card
• Import a transcript
• Save a roadmap while logged out

We should follow a very similar implementation and structure to Scheduler. For now, we can keep autocapture on, but will likely disable it once we get better custom event coverage.