Feature/stripe backend

[jshearer]

Description:

Implements the backend for billing details using stripe's API:

• Create SetupIntents that validate and store a payment method for future use
  • When a tenant first tries to create a SetupIntent, we also create a Customer in stripe for that tenant
• List payment methods
• Delete a payment method
• Switch your primary payment method

There is also basic authorization to ensure that the user making the network request has admin on the tenant they're making the request for. In the future we should probably have a separate billing role, or something like that

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This change is 

[psFried]

LTGM