Make the "load image" text field respond to pressing enter

Summary:
Cam has been using the feature a lot, and really wanted to be
able to press enter to load stuff instead of having to acually click the
button.

Test Plan:
 - Load the page
 - Type in a web+graphie:// link (like
   `web+graphie://ka-perseus-graphie.s3.amazonaws.com/6807c499c78d2a58280a3f177cce384749c52f0c`)
 - Press enter
 - See that it loads

Reviewers: eater

Reviewed By: eater

Differential Revision: https://phabricator.khanacademy.org/D15016