Handle backend updates with correct timezone

[emibcn]

Currently, the data auto update mechanism only works if the user has the same timezone than the official at Catalonia: CEST (with timelight.

https://github.com/emibcn/covid/blob/master/app/src/Backend/Maps/index.js#L132

  // Calculates haw many milliseconds until next schedulled update (today's or tomorrow)
  // TODO: Take care of timezones: Official date is in CEST/GMT+0200 (with daylight saving modifications), Date uses user's timezone and returns UTC
  //       Now, it only works if user timezone is CEST
  //       Probably, the best would be to translate both dates into UTC and, only then, compare them
  millisToNextUpdate = () => {
    const now = new Date();
    const todayDataSchedule = new Date(now.getFullYear(), now.getMonth(), now.getDate(), ...this.officialUpdateTime, 0, 0);
    const millisTillSchedulle = todayDataSchedule - now;

    return millisTillSchedulle <= 0
      ? millisTillSchedulle + 86_400_000 // it's on or after today's schedule, try next schedule tomorrow.
      : millisTillSchedulle
  }