(core) add users.options.isConsultant flag, and omit such users from billing

Summary:
This adds an optional `isConsultant` flag to `users.options`, and an endpoint that allows the support user to turn it on or off. Users marked as consultants are not counted as billable members. Follows the example of existing `allowGoogleLogin` option.

Billable members are counted when members are added or removed from a site. Changing the `isConsultant` flag has no immediate or retroactive effect on billing. The number of users in stripe is now set unconditionally, rather than only when it has changed.

Notifications to billing managers are not aware of this billing nuance, but continue to report user counts that include consultants. The notifications link users to the billing page.

Test Plan: extended test

Reviewers: georgegevoian

Reviewed By: georgegevoian

Subscribers: anaisconce, jarek

Differential Revision: https://phab.getgrist.com/D3362

app/client/ui/BillingPage.ts

@@ -174,7 +174,7 @@ export class BillingPage extends Disposable {
           ] : null,
           moneyPlan.amount ? [
             makeSummaryFeature([`Your team site has `, `${sub.userCount}`,
-                                ` member${sub.userCount > 1 ? 's' : ''}`]),
+                                ` member${sub.userCount !== 1 ? 's' : ''}`]),
             tier ? this.buildAppSumoPlanNotes(discountName!) : null,
             // Currently the subtotal is misleading and scary when tiers are in effect.
             // In this case, for now, just report what will be invoiced.