Media Grid/Modal Keyboard navigation improvements:

* Add focus to arrows on Next/Previous in the grid's modal on left/right keypress, and add the necessary CSS for `:focus`
* When in a disabled input in the grid modal, allow the left/right keys to work
* Make the image editor return a `$.Deferred` so that there isn't a race condition with UI loading. 
* Assign focus when the edit image mode is rendered so that the modal can be closed on Esc press

Props wonderboymusic, adamsilverstein (for the initial patch).
See #23560.

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git-svn-id: http://core.svn.wordpress.org/trunk@29335 1a063a9b-81f0-0310-95a4-ce76da25c4cd