Add $wgSoftBlockRanges

Add $wgSoftBlockRanges

This variable allows for blocking anonymous contributions from certain
IP addresses. Account creation from these addresses will be allowed.

The idea here is that, for example, Wikimedia could add 10.0.0.0/8 to
prevent logged-out bots on labs from making confusing edits. See
I74f5f4a3.

The default for the new variable is empty to avoid causing issues on
upgrade for wikis on private networks.

Change-Id: I6c11a6b9e1a740de074e7ccd753418f94c4b6288