Use per-campaign buckets, with smooth transition

Introduces a new global, $wgCentralNoticePerCampaignBucketExtension,
which indicates the time in days to to keep per-campaign buckets
after a campaign has ended.

Change-Id: I74feda519890a93deae6184bff76f39c741e506c
Merge "Use per-campaign buckets, with smooth transition"