Fix svg-image styling so it works in inline environments

Summary:
In radios, all of the body of the choices is rendered inline.
Svg-image assumed it was in a block, which broke it. This changes its
styling so that it is rendered as an inline-block, so it should work in
any environment.

Test Plan:
 - Make a multiple-choice widget
 - Add a markdown link to a graphie+svg image
 - Ensure that the image displays correctly, takes up the right amount
   of space, and doesn't flow into the next choice.
 - Do the same thing in IE 9, IE 11, and Firefox.

Reviewers: alex, aria

Reviewed By: aria

Differential Revision: https://phabricator.khanacademy.org/D14923