Add a cap to video queue to avoid excessive wait times during heavy usage

[Keith Hughitt]

A hard-limit on the number of videos in the queue should be put in place. By placing such a limit and making is fairly low (e.g. 10-20 videos) it will ensure that any requests that are submitted will be processed within a reasonable amount of time, and avoid having a queue that grows at a much faster rate than the movies can be processed.

As more resources become available to Helioviewer.org for processing movie requests, the queue limit can be increased.