Correctly handle 'now' and wrong advance reservations in job model

The job model was parsing the input job reservation date before passing it to the internal OAR API. Parsing exceptions were caught to return nil, so if the date was not parseable ('now' included which is a valid oarsub keyword for advance reservations), resulting of job created as batch and not advance reservations.

This is now fixed by not parsing the date, and by using the regex used by oarsub to validate the advance reservation format (including 'now'). If the format is not valid, an error is returned.

Documentation was also updated to match oarsub behavior.