Drop support for verification of detached cleartext signatures (#1265)

(Also, use turnstyle to avoid CI browserstack tasks running in parallel.)

.github/workflows/browserstack.yml

@@ -14,6 +14,12 @@ jobs:
 
     steps:
     - uses: actions/checkout@v2
+    - uses: softprops/turnstyle@v1
+      with:
+        poll-interval-seconds: 30
+        abort-after-seconds: 900
+      env:
+        GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
     - uses: actions/setup-node@v1
     - run: npm ci
     - run: npm run build-test
@@ -28,6 +34,12 @@ jobs:
 
     steps:
     - uses: actions/checkout@v2
+    - uses: softprops/turnstyle@v1
+      with:
+        poll-interval-seconds: 30
+        abort-after-seconds: 900
+      env:
+        GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
     - uses: actions/setup-node@v1
     - run: npm ci
     - run: npm run build-test --lightweight