Consolidate all Danger workflows into one

See discussion here:
wordpress-mobile#13476 (comment)

.github/workflows/danger-prs.yml → .github/workflows/danger.yml

@@ -1,14 +1,16 @@
-name: iOS Consistency Checks
+name: Run Checks with Danger
 on:
   pull_request:
-    # The restrictions on the types of pull_request event that can trigger this
-    # workflow is intentional. We only want to run these checks when the PR is
-    # opened or reopened, or its branch receives a push.
-    types: [opened, reopened, synchronize]
+    # Because we have a rule that validates the PR labels, we want it to run
+    # when the labels change, not only when a PR is opened/reopened or changes
+    # are pushed to it.
+    types: [opened, reopened, synchronize, labeled, unlabeled]
 
 jobs:
-  check:
+  danger:
     runs-on: ubuntu-latest
+    env:
+      GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
     steps:
       - uses: actions/checkout@v2
 
@@ -32,10 +34,14 @@ jobs:
         # frozen-lockfile will make the build fail if the lockfile is not there
         run: yarn install --frozen-lockfile
 
-      - name: Check
+      - name: Validate Labels
+        run: |
+          yarn run danger ci \
+            --dangerfile Automattic/peril-settings/org/pr/label.ts \
+            --id pr_labels
+
+      - name: Consistency Checks
         run: |
           yarn run danger ci \
             --dangerfile Automattic/peril-settings/org/pr/ios-macos.ts \
             --id consistency_checks
-        env:
-          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}