Added assets.json environment override for including custom css and j…

[purcebr]

…s files.

What does this PR cover?

Allowing an assets.json file to override the assets portion of data.json

How can this be tested?

Add an assets.json file with the format:

{ "css": [], "js":[] }

Screenshots / Screencast

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I added a minified version in, not sure if that's something you before a release? Let me know if you need any thing else from me. I can merge it into the 2.0 branch, too - I remember you mentioned that we'd need to do that.