create domain/delete-session function

[joyheron]

A rough sketch of the spacy.domain/delete-session function:

(defn delete-session [state _current-user {:keys [id]}]
  {:post [(s/valid? ::outcome %)]}
  {::facts [#_(some new fact containing at least the session id)]
   ::event (-> state
               #_(update function which deletes session from schedule and waiting-queue ))}))