Config setting for cache duration of entity data.

This introduces $wgWBRepoSettings['dataSquidMaxage'] instead
of using $wgSquidMaxage to determine the cache duration for
the output of Special:EntityData. $wgWBRepoSettings['dataSquidMaxage']
defaults to $wgSquidMaxage, but should be overwritten when and where
$wgSquidMaxage is larger than a few hours, since entity data isn't
automatically purged from HTTP caches.

Change-Id: I1dabe792619c5ebd690bfe3e7bf5cd28eb5f19c9
Merge "Config setting for cache duration of entity data."