Limit the storage of data to 30 days.

    Because the graph would become too big, and also, older data is
    quite irrelevant I decided to limit the amount of data stored in
    localStorage to 30 days. All data older than that will be erased.

    Every time a new day is pushed to the "days" list, the length of the
    arrays is verifed and if it's equal to 30, than the first key (the
    oldest) is deleted and after that the current day is added at the
    end of the array.

    The graph will show only the data of the last 30 days.