Clean version management with generated version.js

- Replace package.json modification with generated src/version.js
- App imports VERSION from ./version instead of package.json
- version.js is auto-generated during prebuild and git-ignored
- Provides VERSION, IS_RELEASE, and BUILD_TIME constants
- No more dirty git status from version changes

Development builds show 1.0.4-dev, tagged releases show 1.0.4

package.json

@@ -5,6 +5,7 @@
   "main": "index.js",
   "scripts": {
     "start": "react-scripts start",
+    "prebuild": "node scripts/version-check.js",
     "build": "react-scripts build",
     "test": "react-scripts test",
     "server": "node server.js"