Segregate right to edit sitewide CSS/JS
To limit the likelihood of incidents like T189665, the ability to
edit sitewide CSS/JSON/JS is split out from editinterface, into
separate 'editsitecss', 'editsitejson' and 'editsitejs' rights.
editsitecss, editsitejs, and the right to edit another (potentially
more privileged) user's personal CSS/JS is removed from sysops, and
a new user group, interface-admin, is created specifically for that
task (along with a new grant group 'editsiteconfig').
interface-admin is granted to the first user of a new wiki, along with
sysop and bureaucrat.