Add $wgHideUserContribLimit config setting

Add $wgHideUserContribLimit config setting

Replace const in Special:Block by a config setting in
DefaultSettings.php
Setting to false will disable the limit (instead of setting to a
very high number).

The value is unchanged, but now it is possible to change it in
LocalSettings.php
Also add the value to the message to give better feedback to the
performer of the action.

Change-Id: I3fd39c1c9dfa65a24a3451800dc623b40162aeb5