Fix default for $wgAbuseFilterAnonBlockDuration

$wgAbuseFilterAnonBlockDuration didn't default to the actual
value of $wgAbuseFilterBlockDuration but to the default value
of that (which is indefinite). Fixed that (untested)

Change-Id: I26a929bfba997b80445a108e212030fe7faa6428
Merge "Fix default for $wgAbuseFilterAnonBlockDuration"