Add workflow to upload sentry sourcemaps (#19480)

On push to develop, run ci_package.sh setting SENTRY_ environment variables, which causes webpack to invoke the sentry plugin to upload sourcemaps

.github/workflows/sentry-sourcemaps.yaml

@@ -0,0 +1,24 @@
+name: Upload Sentry Sourcemaps
+on:
+  push:
+    branches:
+      - develop
+
+jobs:
+    runs-on: ubuntu-latest
+    environment: develop
+    steps:
+        -   uses: actions/checkout@v2
+        -   uses: actions/setup-node@v2
+            with:
+                node-version: '14'
+                cache: 'yarn'
+        -   run: ./scripts/fetch-develop.deps.sh --depth 1
+        -   run: yarn install
+        -   run: ./scripts/ci_package.sh
+            env:
+                SENTRY_AUTH_TOKEN: ${{ secrets.SENTRY_AUTH_TOKEN }}
+                SENTRY_DSN: ${{ secrets.SENTRY_DSN }}
+                SENTRY_URL: ${{ secrets.SENTRY_URL }}
+                SENTRY_ORG: sentry
+                SENTRY_PROJECT: riot-web