Don't use UNIX timestamp for wgNoticeOldCookieApocalypse config

Follows-up e12449c53a3.

In MediaWiki settings are loaded before MediaWiki has normalised
the timezone in php (since one of the config vars is for timezone).

This causes warnings like:
> PHP Warning:  strtotime(): It is not safe to rely on the system's timezone
> settings. You are *required* to use the date.timezone setting or the
> date_default_timezone_set() function. Defaulting to 'UTC'.

Instead, similar to $wgCacheEpoch and $wgThumbnailEpoch in
MediaWiki core, set config to MediaWiki-style timestamps and
parse at run time.

Change-Id: I16f7482ea92dfef8bdceb550e6b8c508cc5fa0cf