Modify certificates_show_before_end_date behavior

Hide the certificate notification box when there's
no certificate information, even when the flag
certificates_show_before_end_date is turned on.

ECOM-11
Move certificates_show_before_end into a new variable

Mark it as deprecated, but maintain backwards compatibility.

ECOM-11
Merge pull request #4764 from edx/diana/show-cert-without-end-date

Show Certificates Before End