Billing page improvements

[paolodamico]

Changes

This PR introduces a bunch of changes to improve the self-serve billing page for cloud customers.

• Provide the user with more information around their current plan and allow them to manage their subscription.
  

• They can now manage their subscription (we leverage Stripe's customer billing portal) where users can:

  • Cancel their billing subscription
  • Update their card information
  • Update their billing information for invoices (e.g. address)
  • View their past invoices
  

• Clarification around current usage (i.e. events not billed yet) and exact number of events.
  

• Removes the hide-billing-toolbar feature flag as this was intended for our internal team, but we want to dogfood the billing experience too.

• Introduce support for default event allowance prior to setting up a billing agreement (i.e. events you can have for free without entering your card information).

• Related to the above, we will now show an alert on every page if the organization has exceeded 85% of the allocated events in the current month. This will apply for all organizations without a billing agreement, organizations in the startup plan, and organizations in the legacy plans (starter & growth).

  

• Plans in the self-serve billing enrollment page now include all the plan details. Plan details are stored in the database and support HTML (using dangerouslySetInnerHTML).

  

• Makes it clear when you haven't finished set up your billing agreement and adds a clear step to fix it.

  

Checklist

• All querysets/queries filter by Organization, by Team, and by User
• Django backend tests
• Jest frontend tests
• Cypress end-to-end tests

[paolodamico]

Tagging @jamesefhawkins, @piemets & @yakkomajuri in case you want to check this out from UX standpoint.

[paolodamico]

Ping @Twixes. Also let me know if you prefer someone else take a look :)

[yakkomajuri]

Quickly skimmed the code and looked at the screenshots you provided. UI seems good to me and only added one super nit to the code review which is absolutely non-blocking

[paolodamico]

All feedback has been addressed and complimenting PR merged. Could I trouble you with a final look before I merge?