Remove superfluous plug-in reference
Allow to extend list of scripts by plug-ins

Some plug-ins, e.g. OpenProject Impermanent Memberships, utilize
JavaScript to extend the memberships view. After an AJAX request
these modifications need to be re-applied. This refactoring allows
plug-ins to extend the list of functions applied after an AJAX request.
Support stubs in steps
Merge pull request #1364 from opf/release/fix/7772_membership_duration_column_disappears_after_extending

[Release][Fix] 7772 membership duration column disappears after extending