configService for reading config from REST instead of local .json
Automatically generated constants.js to .gitignore
Grunt: build app for production or development env
Ugly start-production script to npm

Will replace later by Grunt.
Use generated constants.js to get API endpoint
Don't verify getting client secret from config

It won't be there.
Frontend URI to generated constants.js
Use ENV constants and config service instead of local config.json
Removed unused config.json
Merge branch 'config_service'