Submission step unavailable and Display Timezone UTC

[TIM-444](https://edx-wiki.atlassian.net/browse/TIM-444)
[TIM-447](https://edx-wiki.atlassian.net/browse/TIM-447)

For the initial release, we assumed that the submission start date and the problem release date were always the same.  Last week, to fix an issue with problem date validation, I introduced a `submission_start` attribute to allow course authors to specify the release date of the submission step.  However, we did not have a template for showing that the submission is unavailable (we showed it as closed, as Jennifer noted in her bug report).

In this PR, I've:
1) Added a template for showing that submissions are unavailable, because the submission start date is in the future.
2) Format dates in the templates to always display the timezone as UTC.
3) Refactored the submission rendering step and added comprehensive tests for the different possible states.

I've also pulled in the changes from #258, mostly to avoid accidentally reverting them (again).

@stephensanchez