Create variable to set length of IP blocks

This creates a $wgAbuseFilterAnonBlockDuration, which
allows for IP blocks to have a different length.

For backwards compatability this is default set to
$wgAbuseFilterBlockDuration.

Change-Id: Ibfd5c9639317150442f745a5759f3c34b38de274
Merge "Create variable to set length of IP blocks"