refreshing expired tokens

Downscoped tokens only live for about an hour.

So when viewing a long video — especially if you pause it for a while and come back later — the viewer stops with an error message, and the `load` event handler is sent an error like this:

> `Shaka error. Code = 1001, Category = 1, Severity = 2, Data = https://public.boxcloud.com/api/2.0/internal_files/269427057885/versions/283639980141/representations/dash/content/audio/0/292.m4s?access_token=1!big...Long...token&box_client_name=box-content-preview&box_client_version=1.24.0,401,,[object Object]`

when Shaka's request for that next audio or video segment is denied with a 401 response.

(The players for other document types may have similar issues, but video is the one we're running into this issue with on a daily basis.)

To address this we need a few things:

1. Know when it's time to replace the token, by
a. detecting a "token expired" failure response when it happens, or
b. learning up-front what the token's lifetime is when it's first provided to the Preview, or
c. given a token, use some Box "tell me about this token" API to determine its expiration time.
2. Have some way to update the token used by the Preview.
* Turns out this already exists! Preview has an `updateToken` method. Also, `accessToken` may be a Promise-returning function, in which case it might not need to do `updateToken` at all, just re-get using the already provided generator.
* but that documentation isn't published anywhere, which I'll file under issue #641