message: use firebase's own types for payload, return a string from a send call
diff: import admin from 'firebase-admin';
  -
  -interface FCMPayload {
  - message: {
  - notification: {
  - body: string;
  - title: string;
  - data: any; // TODO: Define deep link
  - };
  - };
  - token: string;
  -}
  +import { TokenMessage } from 'firebase-admin/lib/messaging/messaging-api';
  export default class FirebasePushNotifications {
  - async send(payload: FCMPayload) {
  - await admin.messaging().send(payload);
  + async send(payload: TokenMessage) {
  + return await admin.messaging().send(payload);
  }
  }
  