(core) Billing for formula assistant

Summary:
Adding limits for AI calls and connecting those limits with a Stripe Account.

- New table in homedb called `limits`
- All calls to the AI are not routed through DocApi and measured.
- All products now contain a special key `assistantLimit`, with a default value 0
- Limit is reset every time the subscription has changed its period
- The billing page is updated with two new options that describe the AI plan
- There is a new popup that allows the user to upgrade to a higher plan
- Tiers are read directly from the Stripe product with a volume pricing model

Test Plan: Updated and added

Reviewers: georgegevoian, paulfitz

Reviewed By: georgegevoian

Subscribers: dsagal

Differential Revision: https://phab.getgrist.com/D3907

app/common/ApiError.ts

@@ -2,15 +2,17 @@
  * A tip for fixing an error.
  */
 export interface ApiTip {
-  action: 'add-members' | 'upgrade' |'ask-for-help';
+  action: 'add-members' | 'upgrade' | 'ask-for-help' | 'manage';
   message: string;
 }
 
+export type LimitType = 'collaborators' | 'docs' | 'workspaces' | 'assistant';
+
 /**
  * Documentation of a limit relevant to an API error.
  */
 export interface ApiLimit {
-  quantity: 'collaborators' | 'docs' | 'workspaces';  // what are we counting
+  quantity: LimitType;  // what are we counting
   subquantity?: string;    // a nuance to what we are counting
   maximum: number;         // maximum allowed
   value: number;           // current value of quantity for user