Update web builds

- Add concurrency directive to prevent simultaneous builds for a single
  branch.
- Add a workflow to delete the web preview when a PR is closed.

.github/workflows/delete-web-preview.yml

@@ -0,0 +1,24 @@
+name: Delete Preview Build
+
+on:
+  pull_request:
+    types: [closed]
+
+permissions:
+  contents: read
+  id-token: write
+  pages: write
+  pull-requests: write
+
+jobs:
+  azure_blob:
+    runs-on: ubuntu-latest
+    env:
+      AZURE_STORAGE_ACCOUNT: mappingseditor
+      AZURE_STORAGE_SAS_TOKEN: ${{ secrets.AZURE_SAS_TOKEN }}
+      BRANCH_NAME: ${{ github.head_ref || github.ref_name }}
+    steps:
+      - name: Install Azure CLI
+        run: curl -sL https://aka.ms/InstallAzureCLIDeb | sudo bash
+      - name: Delete old blobs
+        run: az storage blob delete-batch -s '$web' --pattern "$BRANCH_NAME/*"

.github/workflows/editor-build.yml

@@ -5,6 +5,19 @@ on:
     branches: [main]
   pull_request:
 
+# If another web build starts for the same branch, cancel the previous build. This
+# protects us from two builds trying to upload at the same time and clobbering each
+# other.
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+permissions:
+  contents: read
+  id-token: write
+  pages: write
+  pull-requests: write
+
 jobs:
   mappings_editor_build:
     runs-on: ubuntu-latest