Fix for #5300: prevent email reports from being sent twice

This is a hotfix for #5300 to prevent email reports from being sent more than once.

Now each time a report is sent, the last "period" it was sent for will be stored in the Option table. Before sending a report, we check if we have already sent a report for the same period.

If the report is sent manually through the web interface, a `force` parameter is set to `true`, which means that the whole "prevent from sending it twice" thing is ignored.
maybe you could write `By default Piwik ensures that a given report is sent only once for each period. Setting this to true will skip this and send the report every time` ?