Add support for GitHub Actions

Build PRs pointing to dev or master branch and create a comment linking to the APK download page.

.github/workflows/build-pull-requests.yml

@@ -0,0 +1,34 @@
+name: Build Pull Requests
+
+on:
+  pull_request:
+    branches: # we might want to add all branches here
+      - 'master'
+      - 'dev'
+
+jobs:
+  build:
+    name: Build project & generate APK
+    runs-on: ubuntu-18.04
+    steps:
+      - uses: actions/checkout@v1
+
+      - name: set up JDK 1.8
+        uses: actions/setup-java@v1.4.0
+        with:
+          java-version: 1.8
+
+      - name: Build debug APK
+        run: bash ./gradlew assembleDebug --stacktrace
+
+      - name: Upload APK
+        uses: actions/upload-artifact@v2
+        with:
+          name: app
+          path: app/build/outputs/apk/debug/*.apk
+
+      - name: Create comment linking to the artifact
+        uses: thollander/actions-comment-pull-request@1f25fabed60c3f141743c3a522529950a8fb2191
+        with:
+          message: 'The APK was build successfuly. You can find it here: https://github.com/TobiGr/NewPipe/actions/runs/${{github.run_id}}'
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}