add github action to build ios/android app

(trying out flutter github actions)

.github/workflows/flutter-actions.yml

@@ -0,0 +1,23 @@
+name: build-app
+
+on:
+  workflow_dispatch:
+  push:
+    branches: [ master ]
+  pull_request:
+    branches: [ master ]
+
+jobs:
+  build:
+    runs-on: macos-latest
+    steps:
+    - uses: actions/checkout@v1
+    - uses: actions/setup-java@v1
+      with:
+        java-version: '12.x'
+    - uses: subosito/flutter-action@v1
+      with:
+        channel: beta
+    - run: flutter pub get
+    - run: flutter build ios --release --no-codesign
+    - run: flutter build apk