workflows: initial user UI integration

* Kickstarts the user UI integration of Holding Pen by adding
  a new menu item "Your Pending Actions" to the user UI and new
  settings pages.

* Refactors the initial index page of Holding Pen to more nicely show
  pending actions and button to take users to the Holding Pen.

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

* Improves the procedure of getting workflow definition objects
  to be compatible with older workflows that may not exist
  anymore and at the same time be more forgiving.

* Fixes some documentation typos.

Signed-off-by: Jan Aage Lavik <jan.age.lavik@cern.ch>
workflows: widget->action name change

* Changes the naming of Holding Pen "widgets" to "actions" to
  more clearly distinguish these UI elements from widgets elsewhere
  in Invenio. The name "action" better suits these types of UI
  components.

* Renames several files to make use of this new naming strategy.

Co-authored-by: Guillaume Lastecoueres <guillaume.lastecoueres@cern.ch>
Signed-off-by: Jan Aage Lavik <jan.age.lavik@cern.ch>
base: kwalitee fixes

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