Gracefully handle Studio authors setting invalid problem dates

[TIM-423](https://edx-wiki.atlassian.net/browse/TIM-423)  See the docstrings and comments in this PR for the reasoning behind this solution.

Some implications of this approach:

1) Course authors will no longer be able to create an invalid date configuration by setting inherited start/due dates.
2) Existing problems will continue to work.  This relaxes a validation requirement without introducing additional constraints.
3) If a course author sets a submission deadline *after* the problem deadline, the problem will stay open until the submission deadline.  I think this is the right solution, since if the course author explicitly sets the submission deadline in the problem definition, that's probably the deadline they want.

@stephensanchez Please review.  This isn't super urgent, since there is a workaround, but I'd like to get a solution to this merged before too many course authors start creating problems.

@Lyla-Fischer  I'm interested in your thoughts about this solution.  Happy to discuss the root cause of the bug and the nuances of this approach.