Update mixed targets for an add-on

Replaces the add-on's full target list with the supplied users, groups, and/or organisations. Set setTargetsForAllVersions=true to apply the same target list to every published version of the add-on; otherwise only the current version is updated. Targets that drop out of the list lose access on subsequent installs but keep existing installations — clean those up via GET /validateTarget/group/ and /user/.