CI: Publish releases on Flathub

Now that Flatpak has achieved full feature parity with non-Flatpak
builds, it is time to publish it directly into Flathub.

Add a new "publish" job to the Flatpak workflow that builds and
publishes to Flathub Beta, and to Flathub (if the release tag
doesn't contain '-rc').

.github/workflows/flatpak.yml

@@ -9,6 +9,9 @@ on:
   pull_request:
     paths-ignore: ['**.md']
     branches: [master]
+  release:
+    types: [published]
+    branches: [master]
 
 env:
   TWITCH_CLIENTID: ${{ secrets.TWITCH_CLIENT_ID }}
@@ -24,6 +27,7 @@ jobs:
   generate_bundle:
     name: Generate Flatpak Bundle
     runs-on: [ubuntu-latest]
+    if: github.event_name != 'release'
     container:
       image: bilelmoussaoui/flatpak-github-actions:kde-5.15-21.08
       options: --privileged
@@ -53,3 +57,58 @@ jobs:
           bundle: obs-studio-${{ github.sha }}.flatpak
           manifest-path: CI/flatpak/com.obsproject.Studio.json
           cache-key: flatpak-builder-${{ github.sha }}
+
+  publish:
+    name: Publish to Flathub
+    runs-on: [ubuntu-latest]
+    if: github.event_name == 'release'
+    env:
+      FLATPAK_BUILD_PATH: flatpak_app/files/share
+    container:
+      image: bilelmoussaoui/flatpak-github-actions:kde-5.15-21.08
+      options: --privileged
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v2.3.3
+        with:
+          submodules: 'recursive'
+
+      - name: Build Flatpak Manifest
+        uses: bilelmoussaoui/flatpak-github-actions/flatpak-builder@master
+        with:
+          bundle: obs-studio-${{ github.sha }}.flatpak
+          manifest-path: CI/flatpak/com.obsproject.Studio.json
+          cache-key: flatpak-builder-${{ github.sha }}
+          mirror-screenshots-url: https://dl.flathub.org/repo/screenshots
+
+      - name: Validate AppStream
+        shell: bash
+        working-directory: ${{ env.FLATPAK_BUILD_PATH }}
+        run: |
+          appstream-util validate appdata/com.obsproject.Studio.appdata.xml
+
+      - name: Verify icon and metadata in app-info
+        shell: bash
+        working-directory: ${{ env.FLATPAK_BUILD_PATH }}
+        run: |
+          test -f app-info/icons/flatpak/128x128/com.obsproject.Studio.png || { echo "Missing 128x128 icon in app-info" ; exit 1; }
+          test -f app-info/xmls/com.obsproject.Studio.xml.gz || { echo "Missing com.obsproject.Studio.xml.gz in app-info" ; exit 1; }
+
+      - name: Commit screenshots to the OSTree repository
+        run: |
+          ostree commit --repo=repo --canonical-permissions --branch=screenshots/x86_64 flatpak_app/screenshots
+
+      - name: "Publish to Flathub Beta"
+        uses: bilelmoussaoui/flatpak-github-actions/flat-manager@v4
+        with:
+          flat-manager-url: https://hub.flathub.org/
+          repository: beta
+          token: ${{ secrets.FLATHUB_BETA_TOKEN }}
+
+      - name: "Publish to Flathub"
+        uses: bilelmoussaoui/flatpak-github-actions/flat-manager@v4
+        if: "!contains(github.ref, '-rc')"
+        with:
+          flat-manager-url: https://hub.flathub.org/
+          repository: stable
+          token: ${{ secrets.FLATHUB_TOKEN }}