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.
Plus, I don't think Studio has been internationalized yet anyway.