workflows: actions JS loading

* Makes the JavaScript assets loading for actions separated from
  the general assets loading to allow for easier additions of
  actions on overlays.

Signed-off-by: Jan Aage Lavik <jan.age.lavik@cern.ch>
workflows: documentation enrichment

* Enriches the documentation of the Workflows module by adding
  more information on actions and the Holding Pen.

Signed-off-by: Jan Aage Lavik <jan.age.lavik@cern.ch>