Update closure compiler externals URL & tweak CSS for cluster icons.

[acornelissen]

• Updated the closure compiler externals URL to point to its new location on github.
• Added 'background-size' property to CSS for cluster icons to allow for a single image to be used (with the size set by CSS). This also allows for nice big images that render well on higher-resolution displays.

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[acornelissen]

Signed!

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