Show download size, potential disk space saving, from video download page.

This PR addresses #590 and a few other issues:

Code cleanup:
* Generalizes some functions in `update_videos.js`
* Creates a `softload_json` function, to remove a lot of duplicate code in loading / error handling json files.

Feature dev:
* Create a script (`compute_video_sizes`) that builds a database of video file sizes, that can be shared across installs to estimate expected total download size
* Mark local and expected file sizes on the annotated topic tree, and display totals in the `Update Video` page.

Testing:
* Tested relevant commands that were cleaned up to use `softload_json`
* Tested script `compute_video_sizes`
* Tested UI for showing estimated total download size, and actual total deletion size.

Notes:
* If someone could add info about the current amount of disk space on the volume where `CONTENT_ROOT` resides, that would be great!

![image](https://f.cloud.github.com/assets/4072455/1750113/0d3233a4-656c-11e3-9fa6-7543ab66a9a9.png)

@aronasorman try it out?