Change Modelling task API

[AVGP]

Original issue: https://task.archilogic.com/issues/9776

After the discussion with Florian and Tomas on 27.1.: * Web Server and Redmine user accounts for modellers should correspond: use UUID and resource_name * Get rid of ModellingTask.list * Claiming tasks would claim the top available modelling task directly and return the full claimed ModellingTask as a response * CompleteTask API method would take modelling time as an argument (modellers have a timer) * Add API methods for ** Rejecting (along with Enum of reasons) ** Cancelling a modelling task (return to status Open and no assignee)