So that sysadmins can further customize the extension. It was also wrong
to use the same variable for many different things.
Note that there's no associated patch in wmf-config because we use the
defaults. However, before merging this patch, please recheck that
AbuseFilterRestrictions and AbuseFilterDisallowGlobalLocalBlocks aren't
used there (https://codesearch.wmflabs.org/operations/?q=AbuseFilterDisallowGlobalLocalBlocks%7CAbuseFilterRestrictions&i=nope&files=&repos=)
Bug: T175221
Change-Id: I7581b3ee6d9d11a6cf1599b8ff874e8c3d54adf4
