Grant all gadgets-*** userrights to sysop group by default

Extensions should use a sane default configuration, and it is
expected that sysops will be able to create/edit gadgets. This
is also needed for backwards-compatability from Gadgets 1.0.

Change-Id: I377e9e9a6477a337baf031b4e802ca58586f6392
See: https://www.mediawiki.org/wiki/Thread:Talk:ResourceLoader/Version_2_Design_Specification/Task_management/NS_GADGET_editing
Merge "Grant all gadgets-*** userrights to sysop group by default" into RL2