Date and time pickers

[ORA-658](https://openedx.atlassian.net/browse/ORA-658)

* Add time fields to the OA editing views
* Use resolve dates for default date values

Companion PR in edx-platform: https://github.com/edx/edx-platform/pull/4416

@stephensanchez Please review.
Just a cleanup thought (from the guy that made the mess in the first place) we may be able to create a common template for all the assessment modules that they extend to get shared fields and config for start/due date (may not be worth it if all the variables are different)