Mobile: Fix issues with development environment

* Install missing jetifier dependency
* Add note about run-android bug to BUILD.md

.gitignore

@@ -40,6 +40,7 @@ Tools/github_oauth_token.txt
 _releases
 ReactNativeClient/lib/csstojs/
 ReactNativeClient/lib/rnInjectedJs/
+ReactNativeClient/android/app/src/main/assets/index.android.bundle
 ElectronClient/app/gui/note-viewer/fonts/
 ElectronClient/app/gui/note-viewer/lib.js
 Tools/commit_hook.txt

BUILD.md

@@ -69,6 +69,10 @@ First you need to setup React Native to build projects with native code. For thi
 
 Then, from `/ReactNativeClient`, run `npm install`, then `react-native run-ios` or `react-native run-android`.
 
+If app crashes with `Unable to load script` error after running `react-native run-android` on Linux, check this StackOverflow question:
+
+https://stackoverflow.com/questions/44446523/unable-to-load-script-from-assets-index-android-bundle-on-windows/44476757
+
 # Building the Terminal application
 
 ```

ReactNativeClient/package.json

@@ -6,7 +6,7 @@
   "private": true,
   "scripts": {
     "start": "node node_modules/react-native/local-cli/cli.js start",
-    "postinstall": "node ../Tools/copycss.js && node ../Tools/buildReactNativeInjectedJs.js && cd .. && npm i"
+    "postinstall": "node ../Tools/copycss.js && node ../Tools/buildReactNativeInjectedJs.js && npx jetify && cd .. && npm i"
   },
   "dependencies": {
     "@react-native-community/slider": "^1.1.3",
@@ -80,6 +80,7 @@
   "devDependencies": {
     "@babel/core": "^7.4.5",
     "@babel/runtime": "^7.4.5",
+    "jetifier": "^1.6.3",
     "metro-react-native-babel-preset": "^0.54.1",
     "react-test-renderer": "^16.8.3"
   }