(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/ActiveDocAPI.ts

@@ -1,5 +1,4 @@
 import {ActionGroup} from 'app/common/ActionGroup';
-import {AssistanceRequest, AssistanceResponse} from 'app/common/AssistancePrompts';
 import {BulkAddRecord, CellValue, TableDataAction, UserAction} from 'app/common/DocActions';
 import {FormulaProperties} from 'app/common/GranularAccessClause';
 import {UIRowId} from 'app/common/TableData';
@@ -320,11 +319,6 @@ export interface ActiveDocAPI {
    */
   getFormulaError(tableId: string, colId: string, rowId: number): Promise<CellValue>;
 
-  /**
-   * Generates a formula code based on the AI suggestions, it also modifies the column and sets it type to a formula.
-   */
-  getAssistance(request: AssistanceRequest): Promise<AssistanceResponse>;
-
   /**
    * Fetch content at a url.
    */