Optimize admin user interface

[Erin Bell]

Use CSS flex for search result grid (to keep things all lined up).

Add some instructions on the settings panel to indicate how the plugin works (I actually missed the widget the first time around and was thinking it would be standalone rather than on the post edit page).

Maybe add some other enhancements to make the widget feel more distinct (it sort of feels like it's hiding below the post).

[Erin Bell]

TODO: load fullsize image src for each item on demand (i.e. when the thickbox is opened) to keep the page load manageable.