fix keyboard shortcuts and help not working on workpackage list

[``* `#8747`keyboard shortcuts and help not working on workpackage list``](http://www.openproject.org/work_packages/8747)

@ulferts @myabc I decided to add another class for speed purposes instead of a data attribute ( I think it's faster than through some [data-list='true'] selector).
@0xF013 aren't those the specs you added with the former pull request where you also introduced the "list" class to the work package list? As this is scoped to the work packages view I guess it is pretty save.