Charge tax on products

[fetchfern]

Integrates with Anrok.

While at it this also massively cleans up the code handling PaymentIntents, and fixes a bug where the intent metadata wasn't being updated when updating the intent via /_internal/billing/payment.

Merging of this is blocked by #4338, so we can implement email notifications before charging people taxes.

Still left to do:

• Actually use the PaymentIntent functionality in the payments module everywhere
• Alert emails
• Pre-fill the database with values for products_tax_identifiers
• Tests

[fetchfern]

May need to do some tweaks to the approach to price change notifications w.r.t taxes + tax calculation pending some clarifications from product but it's 90% of the way there—ready for review.

It's not the cleanest implementation I'll admit but it works and needs to be shipped.

[AlexTMjugador]

In general this looks sensible enough to try out. I couldn't review or test this too deeply, but I suppose it's better than nothing.

[fetchfern]

There was an issue with my approach to notify users about price changes (wouldn't notify users if the sales tax rate in their jurisdiction changes) so I ended up modifying this PR quite a bit and fixed some of the implementation issues with it:

• Generate Anrok transactions through a queue worker rather than within the webhook directly (no more tokio::spawn in an impl Drop!)
• Continuously update the tax amounts rather than just when the tax amount is initially zero to account for sales tax changes, and insert the related notification in the same task rather than a separate one. This removes the need for the user_aware_of_tax_changes column so I removed it.
• More variables available to the tax change notification template.
• Store product names in the DB rather than hardcoding them.
• Retry logic for Anrok API operations with random exponential backoff.

Still waiting on the tax-notification email on prod. Requesting review again since the changes are significant.

[AlexTMjugador]

Nice, this looks noticeably better now! 🎉