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.

Bug: T120886
Bug: T190015
Depends-On: Ia9b2ea1450aff6121dc0f3777bf029292c8aaad9
Change-Id: Ifefd872640642441e26f8b2f144ffe4b88d2eb12