Take redirect modifications into account for If-Modified-Since

When the pageview is following a redirect, MediaWiki is only considering
the page_touched timestamp for the target page. This causes issues with
handling If-Modified-Since if the redirect is retargeted after the
target page was last touched.

The simple solution is to use the maximum page_touched of the page or
the redirect (or any other redirect in the chain, if
$wgMaxRedirects > 1).

Bug: 67849
Change-Id: Ia359aaff6eab34c1d609254055c122a644fa7cce
Merge "Take redirect modifications into account for If-Modified-Since"