Feature: download json and drag/drop it back in

[nsonnad]

PR to track progress of drag/drop functionality for exported chart JSON

Todo:

• [x] Download chart model as a file
• [x] Drag/drop downloaded file back into Chartbuilder, updating current chart
• [x] Validate incoming JSON
• [x] Add test for util/validateChartModel.js: file is not JSON returns error
• [x] Add test for util/validateChartModel.js: file is not a Chartbuilder model returns error
• [x] Add test for util/validateChartModel.js: file is a Chartbuilder model returns parsed JSON
• [x] Bump version number
• [ ] ?? Anything I'm missing