Fix bug 1109289

The only problem with this change is that if an event has a vidly_template but not a vidlysubmission things can go wrong but I don't know if such situation is possible
First of all we can't guarantee that there's just 1 VidlySubmission for this event. There's a reasonably big chance that there might be more than one which is the case when they have to re-submit a new video to Vid.ly due to some encoding problems or something. Also, there's actually a possibility that an event has a Vid.ly template but 0 VidlySubmissions. The only reasonable case I can think of is if someone has cloned an event and manually copied the vidly tag over to the new event. Or perhaps they made the vidly submission manually with some other tool. 

I think the best solution would be to assume that HD is NOT available. That's because I think 404 Page Not Found is more horrible than that a user has to watch the downloaded SD version.

Also, when you do that query on `VidlySubmission.objects.` you should also use the tag. E.g. `event=event, tag=context['vidly_tag'])`. 

So I think the ideal solution would be to use a loop. Like this:

```python
hd = False  # default
vidly_submissions = (
    VidlySubmission.objects
    .filter(event=event, tag=context['vidly_tag'])

)
for vidly_submission in vidly_submissions.order_by('-submission_time'):
    hd = vidly_submission['hd']
    break
```

That way it works if there are 0 or more than 1 VidlySubmissions (by filtering by `tag`) it's extremely unlikely to ever be more than 1 but even if there is we solve that by sorting by submission_time and breaking.