Expose process timestamps on RetrieveProcess

The created, updated, started and finished attributes on Process are only exposed in ListProcesses, not RetrieveProcess.

Since the workflow is no longer present on processes, started is the easiest way to detect whether a process has already started or not in the frontend, and this is exactly how the frontend was updated in frontend!1491 (merged). With started being missing from RetrieveProcess, various checks on the process are no longer working, including the one preventing access to /process/<id>/configure for a process that already started.