MailService.prepareDailyStatistics(): place method to a better place

[php-coder]

MailService.prepareDailyStatistics() has no relation to mail service now. We need to find a better place for it. For example, we can create ReportService for it.

Note that after extracting method to another class, we will have a problem with getting access to it because the method has @PreAuthorize(HasAuthority.VIEW_DAILY_STATS) annotation.

Tech debt for eb57501 (#589)

CC @bodom91 in case you want to work on it