Allow self paced courses to have no upgrade deadline

[annagav]

As of now if a course offers certificate we show the upgrade deadline in the Course info box and in the upgrade dialog.

For self paced courses the upgrade deadline might not make sense.

Current Behavior:

If upgrade deadline is not set on a course we treat it as if upgrade is not possible.

Desired Behavior:

if the course is self paced, has a product, but no upgrade deadline -> allow users to upgrade (and hide any deadline warnings)

[pdpinch]

All of the courses in production currently have an upgrade deadline, so this isn't a high priority fix.

Peter will bring this up with the MITx team to find out how likely it may be that a course doesn't have an upgrade dealine.

[annagav]

If we don't have any implemented UX behavior in case upgrade_deadline is null, then we should make upgrade deadline a required field.