Update banner messaging in billing page after a subscription purchase

After a purchase of a subscription using the in-app checkout flow, once you navigate back to the billing page for the group that just upgraded, it still says you’re on the free plan and has options for starting a trial or purchasing any of the paid tiers.

My hunch is that the page gets cached so that it will load more quickly (since it has to make a call or two to the CustomersDot app), and we should bust that cache in the event of a subscription change (if it is easy enough to do so).

Proposal

↩️ Redirect to the billing page after a successful purchase

When completing the purchase, redirect the user to the billing page (e.g., groups/<group-namespace>/-/billings). See here.

🟢 Update the message in the success banner to account for async operation

Display an additional text in the success banner that says:

Thanks for your purchase!

You have successfully purchased an Ultimate plan subscription for 1 user. You'll receive a receipt via email. Your purchase may take a minute to sync, so refresh the page if you don't see it yet.

Additionally, update the alert variant to be of type success.

Implementation plan

Description	MR
Update success banner to account for async operation	!96220 (merged)