Temporary fix for bucket stickiness and expiry

This is a temporary measure to ensure that buckets do not cycle
haphasardly as described here:
https://www.mediawiki.org/wiki/Extension:CentralNotice/Buckets#Cycling_bucket_cookie_expiries.3F

With this patch, bucket cookies work like this:
- When requesting a banner, get a bucket from the bucket cookie, or
  if none is available, choose one randomly (same functionality as
  before).
- Only store the bucket in a cookie if a banner is received.
- Every time a banner is received, extend the bucket cookie's validity
  to last into the future the time indicated by wgNoticeBucketExpiry.

Change-Id: If520a709856d835ca37d41630329aeefeea466e7
Merge "Temporary fix for bucket stickiness and expiry"