ui-stats: replace 'enable-stats' setting with 'max-stats'

The new 'max-stats' and 'repo.max-stats' settings makes it possible to
define the maximum statistics period, both globally and per repo. Hence,
it is now feasible to allow statistics on repositories with a high commit
frequency, like linux-2.6, by setting repo.max-stats to e.g. 'month'.

Signed-off-by: Lars Hjemli <hjemli@gmail.com>