feature: show global stats (mirrors combines) and add weekly/monthly/yearly per mirror

[MartijnKaijser]

As title says. Currently data usage in mirrorstats is onnly shown per mirror when you hover over it. However it's only current day total so to actually know the full day usage you would have to check at specific time. If possible a counter for current day / month / year would help doing estimations of traffic (same as done for download counts). As extra a global counter for all mirrors combined would be a nice extra

@kib will probably already see what's available and adjust the layout