Add usage section to deployment page in dashboard

[deansheather]

Part of phase 2 of Usage-based Billing RFC

Add a new sidebar section to the Deployment page in the dashboard that embeds a dashboard URL retrieved from Tallyman in an iframe. This will also require an endpoint on coderd to proxy the request to Tallyman, probably something like /api/v2/entitlements/usage/embed

Depends on https://github.com/coder/tallyman/issues/3