Add billing support for Spoke owners

[ajohn25]

Is your feature request related to a problem? Please describe.
Spoke doesn't have any real support for billing, and billing relationships between an organization that manages infrastructure (ex. WTR) and an organization that sends texts (our clients) need to be managed entirely outside the application.

Describe the solution you'd like
A billing system within the application that collects payment information, and notifies Spoke owners of billing related events. Ideally, we want this to get as close as we can to "autopay," so that our involvement with tracking down unpaid invoices etc. is minimal.

Describe alternatives you've considered
Using a third party billing platform without a direct connection to Spoke data as we currently do

Additional context
This should get a design doc for review - this link should give you the markdown template

Some things to consider:

• How will Spoke owners track their usage?
• What occurs when a new invoice/payment is ready? Consider actions in the app and other services if needed
• What occurs when a payment is overdue? Consider actions in the app and other services if needed
• What payment platform will Spoke integrate with? QBO, Stripe, something else?
• What payment methods will we support? ACH, Credit Card, etc. Consider tradeoffs like fees, ease of setting up "autopay", etc.