add contentBase for serving assets from src directory in development mode.

[aranajhonny]

content-base #503 posible solution

[ghost]

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[tal]

As currently designed images in the root directory seem to be copied over like the favicon, i was gonna look into how that's setup. I think it's a cleaner solution to have a public foloder for all static assets.

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[gaearon]

Please also see #226, #413, #428.

I don’t think it’s a good idea to just make everything in src accessible as content because it will also mean we have to copy the entire src directory into the build folder, or builds would be broken.