Consider reporting the sum of website last month stats

After #16956 we now get 2 tails-website-last-month-stats results in tails-dev@ and need to manually sum them in order to get the total. This may become an issue if, for example, we implement more mirrors or if it happens that people consistently forget that they need to sum the numbers. At some point it may make sense to find a way to sum the numbers in the server side before sending the stats report.