(fr #719) all cookie hiding is configurable

Expiration is always calculated as an interval, there are no cutoff dates.

Two configuration variables are introduced for this purpose,
$wgNoticeCookieShortExpiry and $wgNoticeCookieLongExpiry

Change-Id: I291375ef87291e69292af0d0e27f81ee921e07f4
Merge "(fr #719) all cookie hiding is configurable"