Move some core modules to core/storage submodule
Introduce exportAll() function to wrap store method
Introduce Store#importAll() and exportAll() to import/export all store content
Add FileSaver.js dependency
Introduce loadStringFromFile() to read file content
Introduce saveStringToFile() to trigger file download
Implement import/export UI
Use loadStringFromFile() in ImportController
Use saveStringToFile() in ExportController
Extract app.importAllFromJSON() & .exportAllToJSON() from ui
Use Store#importAll() and exportAll() to implement app JSON import/export
Fix timezone issue in tests