Pre dubbed video support: migrating schema, scrubbing code.

This is dependent on #1084 and addresses #233, #1087, and others.  Best to view this PR as commits, as the topic updates are overwhelming otherwise.

This lays the foundation for dubbed video support, by:
* Migrating to a more generic `video_id`, instead of language-specific `youtube_id`, as the foundation for identifying videos in `VideoLog`
* From the topic tree side, migrate app to differentiate between `id`, which are unique identifiers, and `slug`, which is a (probably-unique, human readable) piece usable for building URLs. `id` is set to the slug for exercises and topics, but not for videos (set to the english language youtube ID).

Each of these were accomplished by renaming variables to be consistent, and adding to the `main/models.py` or `khanload`/`topics.json` schemas.

Potential issues:
* Local dataloss is not an issue; a new field was set in the `VideoLog` model, instead of shifting anything.
* Syncing is the major concern, but there should be no issues:
    * UUID calculation migrated from `youtube_id` to `video_id`, but since `video_id` is set to the English video youtube_id, this will be backwards-compatible.
    * When syncing with old machines, the `video_id` field will be eliminated.  This will cause pain for the unlikely case where a user watched a video in English, and also another dubbed video language, one on a newer install, one on the older.  
        * In that case, locally, there will not be collision--the user will have two separate logs (but only able to update one).
        * On the central server, the dubbed video lookup algorithm will notice these are the same, and will merge them.

Testing:
* Extensive testing was done on `videodownload` and `videoscan` via the "Update Video" page, which all look fantastic now
* Plenty of testing on video viewing, playing, saving, related videos from exercises, topic pages, and that all the green progress bubbles show.
* Some testing on coach reports (particularly tabular view and student view)