LP#1791340 Webstaff: Don't backdate when we're not

LP#1791340 Webstaff: Don't backdate when we're not

There was code in the webstaff checkin interface which attempted to
remove the backdate parameter when it wasn't needed, as its mere
presence has a negative effect on the checkin process.  This fail-safe
was not working, however.

Instead, let's build on the same check function used for the display,
so we can be more sure that if the user doesn't see that they are
backdating, the interface will not treat it as a backdate, and
vice-versa.

Signed-off-by: Dan Wells <dbw2@calvin.edu>
Signed-off-by: Galen Charlton <gmc@equinoxinitiative.org>