Add callout at top of editor if subscriber count exceeds plan limits

[benborgers]

No description provided.

[jmduke]

I remove this in https://github.com/buttondown/monorepo/commit/f39c9727f0de1cf9b08179a0b92a399a1e35ae3e#diff-e6ec570c49d15d43173d1f0b516ae521b1262c8eb68306ebb0ad40e37654557a because it was a surprisingly large performance bottleneck. I think we can bring it back by just exposing a new API route (not unlike needs_billing_information, which I deleted like a fool) and calling that in the generateCallouts flow.

[jmduke]

"Why is it a surprisingly large performance bottleneck" → because we have to count every unique subscriber for the account