[NEW FEAT] Add estimated remaining time when the hourly mail merge process is triggered

[ttsukagoshi]

Pull request #164 introduced a new feature/bugfix where the mail merge process (SEND EMAILS and CREATE DRAFTS) that took longer than the execution time limit of postProcessMailMerge, i.e., the apps script's 6-min limit, is handed over to another hourly triggered postProcessMailMerge to work around the Exceeded maximum execution time error.

It would have been more than convenient for users if this second trigger could be executed at a more frequent interval than an hour, but there is an undocumented limitation that recursive time-based triggers created by add-ons must be set at an interval of at least one hour, else it would throw an exception with the following message:

Exception: The recurrence interval for an Add-on trigger must be at least one hour.

To comply with this limitation while maintaining the user experience of the add-on at a certain level, I've broken down the post-process into two steps in the said PR. The first postProcessMailMerge will be triggered on ClockTriggerBuilder.after(milliseconds):

if (!isTimeTriggered) {
  ScriptApp.newTrigger('postProcessMailMerge')
    .timeBased()
    .after(EXECUTE_TRIGGER_AFTER)
    .create();
}

whereas the subsequent postProcessMailMerge, whose time-based trigger will be set when the first round of execution is expected to take longer than the 6-min limit, will be set by ClockTriggerBuilder.everyHours(1) (where 1 is the smallest acceptable number)

else if (debugInfo.exeRounds == 1) {
  ScriptApp.newTrigger('postProcessMailMerge')
    .timeBased()
    .everyHours(1)
    .create();
}

While I believe this is the best possible solution currently available, I understand this 1-hour interval will be frustrating for users who are not aware of this process, as it will keep them waiting throughout the recursive hourly executions. To properly inform the user, I would like to add the estimated remaining time to the MESSAGE.<userLocale>.continuingPostProcess.