(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/UserAPI.ts

@@ -1,5 +1,6 @@
 import {ActionSummary} from 'app/common/ActionSummary';
 import {ApplyUAResult, ForkResult, PermissionDataWithExtraUsers, QueryFilters} from 'app/common/ActiveDocAPI';
+import {AssistanceRequest, AssistanceResponse} from 'app/common/AssistancePrompts';
 import {BaseAPI, IOptions} from 'app/common/BaseAPI';
 import {BillingAPI, BillingAPIImpl} from 'app/common/BillingAPI';
 import {BrowserSettings} from 'app/common/BrowserSettings';
@@ -462,6 +463,8 @@ export interface DocAPI {
   // Update webhook
   updateWebhook(webhook: WebhookUpdate): Promise<void>;
   flushWebhooks(): Promise<void>;
+
+  getAssistance(params: AssistanceRequest): Promise<AssistanceResponse>;
 }
 
 // Operations that are supported by a doc worker.
@@ -1012,6 +1015,13 @@ export class DocAPIImpl extends BaseAPI implements DocAPI {
     return response.data[0];
   }
 
+  public async getAssistance(params: AssistanceRequest): Promise<AssistanceResponse> {
+    return await this.requestJson(`${this._url}/assistant`, {
+      method: 'POST',
+      body: JSON.stringify(params),
+    });
+  }
+
   private _getRecords(tableId: string, endpoint: 'data' | 'records', options?: GetRowsParams): Promise<any> {
     const url = new URL(`${this._url}/tables/${tableId}/${endpoint}`);
     if (options?.filters) {