Update web build

1. Add the concurrency directive to hopefully avoid having any parallel
   builds of the same branch, which could lead to two builds uploading
   web files at the same time.
2. Add a delete-web-preview workflow that runs whenever a PR is closed
   to delete the temporary web build for that branch.

.github/workflows/build-web.yml

@@ -5,6 +5,13 @@ on:
     branches: [main]
   pull_request:
 
+# If another web build starts for the same branch, cancel the previous build. This
+# protects us from two build 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

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

@@ -0,0 +1,24 @@
+name: Build Website
+
+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: mappingsexplorer
+      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/*"