Create offsets for redirects

This should speed up highlighting for pages with many many many redirects
at the cost of a little disk space.

DEPLOYMENT: This requires an in place reindex to take effect.
DEPLOYMENT: Probably worth doing on wikis that we've seen slow prefix
searches.  This is certainly the cause of some of that and might be the
cause of all of it.  Wikis are eswiki and frwiki.  May as well do dewiki
and enwiki too while we're at it.

Change-Id: I1eacfcb41b622b9ed0462acb69cb4a6b96cf8912
(cherry picked from commit dbd557688a449c6991d6248731885925408ded2b)