forms: Support non-US locale on client portal

The staff portal has a feature where the date fields are converted to
YYYY-MM-DD prior to form submission. However, this code was not ported to
the client portal. Therefore, for non-US date formats (d/m/Y for instance)
would not be processed properly by the server and would be nulled prior to
validation.
Merge pull request #1320 from greezybacon/issue/client-portal-date-fields

forms: Support non-US locale on client portal

Reviewed-By: Peter Rotich <peter@osticket.com>