Issue Trends data relative to period start

Until now the Issue Trend reports were aggregating data into hourly,
daily or weekly buckets, depending on the reporting period's duration.

These buckets were built backwards from the current time, which caused
unexpected results as the first set of data can have an associated
timestamp set before the selected period's start time.

Setting the process' start time with the new Period::get_latest_bucket()
method instead of time() causes the buckets to be relative to the
period's start time (with the last bucket potentially smaller than the
normal size).

Fixes 0034885