Add asset model and controller

app/models/assets/asset_image.rb

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+class AssetImage < Asset
+
+end

app/models/assets/asset_javascript.rb

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+class AssetJavascript < Asset
+
+end

app/models/assets/asset_pdf.rb

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+class AssetPdf < Asset
+end

app/models/assets/asset_stylesheet.rb

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+class AssetStylesheet < Asset
+
+end