CSS and layout fixes for image uploader

closes #1673
- dropzone icon doesn't appear when filestorage is false
- proper cursor and no selection on icon
- cleaned up some sass
Merge pull request #1674 from ErisDS/css-fixes

CSS and layout fixes for image uploader
Serving static assets via asset-rack

closes #1405
- adds asset-rack to package for serving assets
- moves all assets previously served via express.static into an asset rack
- handles url generation for statically served assets
- updates the asset-helper for hbs-templates to get the right url
- allows configuration of local-asstets-deployment directory and CDN