Add a compare to selector to dashboards

Feature request

Is your feature request related to a problem?

It's a not a problem but an enhancement

Describe the solution you'd like

The ability to select the compare to period in a dashboard. Obviously you can do this at an insight level but you have to go into the insight to change the compare to data. When you have multiple insights you'd like to compare against, it get time consuming.

Describe alternatives you've considered

I've tried creating duplicate insights with different compare to windows, which is fine but means a lot of duplicate insights.

Additional context

Debug info

- [ ] PostHog Cloud, Debug information: [please copy/paste from https://us.posthog.com/settings/project-details#variables]
- [ ] PostHog Hobby self-hosted with `docker compose`, version/commit: [please provide]
- [ ] PostHog self-hosted with Kubernetes (deprecated, see [`Sunsetting Kubernetes support`](https://posthog.com/blog/sunsetting-helm-support-posthog)), version/commit: [please provide]

No files or tests are named. Begin by locating the dashboard and insight-level compare-period UI, then determine where dashboard state and comparison settings are handled. Done should mean a dashboard-level period selector changes the comparison period for the dashboard's insights without requiring duplicate insights, with tests for the behavior.