Grant wikilambda-abstract-optin to cross-wiki global groups via mediawiki-config.

Mirroring the approach taken for wikilambda-abstract-purge in M0, the wikilambda-abstract-optin right should also be granted to the stewards and global sysops global groups through WMF mediawiki-config, in addition to the default local sysop assignment. The reasoning is the same as for purge: keeping the two Abstract-Content-specific rights consistent in their cross-wiki posture means sysops and cross-wiki responders manage Abstract-Content permissions as a coherent family rather than as scattered one-offs. An inconsistent posture — purge available to global sysops but opt-in gated to local — would create a weird failure mode in which a global responder could mitigate a bad render (via purge) but could not actually remove the opt-in that caused the problem, leaving them in "I can see the problem, I can't fix it" limbo. This is a mediawiki-config change, not a WikiLambda code change; the extension itself only sets the local sysop default.