Introduce a new wikilambda-abstract-optin user right for the opt-in action, assigned by default to the sysop group.

The default assignment is the one we expect every WMF wiki to use initially, but creating the right as a first-class citizen — rather than implicitly checking for sysop at the call site — means individual communities retain the flexibility to reassign it to a different user group through the standard $wgGroupPermissions / $wgRevokePermissions / community-configured groups mechanisms, without any code change on our side. Introducing the right now is cheap; retrofitting it after the first wiki ships with hard-coded sysop checks would be disruptive. Naming it wikilambda-abstract-optin also gives a symmetric pairing with the wikilambda-abstract-purge right introduced in M0's reactive regeneration bullet, so sysops manage Abstract-Content-specific permissions as a coherent group rather than as scattered one-offs.