(bug 43529) Hide private data after election

If any private data (ip, xff, or ua) is in a vote record, hide this
data from administrators after the election has been closed for a
number of days.

$wgSecurePollKeepPrivateInfoDays is used to configure the number of
days, default is 90.

Change-Id: I4de5a563913a840507e640288e679e9f97a9b1c2