Bring back silent image widget sizing.

Summary:
This is almost the same as 8c705f4e747697036366c28ff47fde95ba0dc707, but
with a few changes to avoid breaking image sizing.

Changes still included:
* set up silent plumbing
* isMounted check to fix possible error

Test Plan:
* In the perseus test page:
  - load [1] in an incognito window, see the image when it loads
  - load [1], click into the url field, click out, the image does not
    disappear
  - load [1], add a duplicate image, no image disappears
* In perseus one:
  - switch between items with and without images, see not flash of
    unsaved indicator
  - change the url of an image and see unsaved, save it and don't see
    unsaved

[1] -
http://localhost:9000/test.html#content=%7B%22question%22%3A%7B%22content%22%3A%22%5Cn%5Cn%5B%5B%E2%98%83%20image%201%5D%5D%22%2C%22images%22%3A%7B%7D%2C%22widgets%22%3A%7B%22image%201%22%3A%7B%22type%22%3A%22image%22%2C%22graded%22%3Atrue%2C%22options%22%3A%7B%22title%22%3A%22%22%2C%22range%22%3A%5B%5B0%2C10%5D%2C%5B0%2C10%5D%5D%2C%22box%22%3A%5B0%2C0%5D%2C%22backgroundImage%22%3A%7B%22url%22%3A%22http%3A%2F%2Frain.thecomicseries.com%2Fimages%2Fcomics%2F40%2F59285d9997e060a1bdc73f7211f7e88c478063601.png%22%2C%22width%22%3A0%2C%22height%22%3A0%7D%2C%22labels%22%3A%5B%5D%2C%22caption%22%3A%22%22%7D%2C%22version%22%3A%7B%22major%22%3A0%2C%22minor%22%3A0%7D%7D%7D%7D%2C%22answerArea%22%3A%7B%22type%22%3A%22multiple%22%2C%22options%22%3A%7B%22content%22%3A%22%22%2C%22images%22%3A%7B%7D%2C%22widgets%22%3A%7B%7D%7D%2C%22calculator%22%3Afalse%7D%2C%22itemDataVersion%22%3A%7B%22major%22%3A0%2C%22minor%22%3A1%7D%2C%22hints%22%3A%5B%5D%7D

Reviewers: aria

Differential Revision: http://phabricator.khanacademy.org/D14764