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.  I chose to use UTC everywhere instead of converting to local time based on Jennifer's comment in the JIRA ticket: "I value consistency in how times are displayed to students, so if every other time in the platform is in UTC, I might prefer to display UTC"

3) Refactored the submission rendering step and added comprehensive tests for the different possible states.

4) Updated the test script to allow us to run a subset of the tests.  Glad that we have over 320 tests now, sad that the test suite takes over 14 seconds to run.

I've also pulled in the changes from #258, mostly to avoid accidentally reverting them (again).

@stephensanchez