Allow gallery and embed browse previews to be clicked in.

Previously we were closing these previews when the surrounding page was clicked on,
however this also included clicks w/i the preview itself. This commit checks to see
if the click target is inside a preview container before it closes all the previews
on the page.

Closes #876 

### Before
![accordion-before](https://cloud.githubusercontent.com/assets/96776/4254626/2fa2acb8-3aad-11e4-9493-c77a32cfd3d1.gif)
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### After
![accordion-after](https://cloud.githubusercontent.com/assets/96776/4254625/2f9d5d58-3aad-11e4-8b94-c5123361cb9d.gif)