(core) Improving billing page user experience

Summary:
Improving billing page user experience.
- Updated labels for canceled plan
- Adding option to downgrade from team plan to free team plan
- Updating default name for teamFree plan when it is not available in Stripe
- Minor bug fixes

Test Plan: updated tests

Reviewers: cyprien

Reviewed By: cyprien

Subscribers: cyprien

Differential Revision: https://phab.getgrist.com/D3515

app/client/models/BillingModel.ts

@@ -48,6 +48,9 @@ export interface BillingModel {
   getCustomerPortalUrl(): string;
   // Renews plan (either by opening customer portal or creating Stripe Checkout session)
   renewPlan(): string;
+  // Downgrades team plan. Currently downgrades only from team to team free plan, and
+  // only when plan is cancelled.
+  downgradePlan(planName: string): Promise<void>;
 }
 
 export interface ISubscriptionModel extends IBillingSubscription {
@@ -124,6 +127,10 @@ export class BillingModelImpl extends Disposable implements BillingModel {
     return this._billingAPI.renewPlan();
   }
 
+  public async downgradePlan(planName: string): Promise<void> {
+    await this._billingAPI.downgradePlan(planName);
+  }
+
   public async cancelCurrentPlan() {
     const data = await this._billingAPI.cancelCurrentPlan();
     return data;