AppVeyor: Set up release-signing by SignPath

Only enabled for development snapshots and tagged releases.

This should produce signed installers at SignPath. I'm told they need to
be manually approved there for now, but in the future hopefully they
could be uploaded automatically instead of the unsigned installers.

(cherry picked from commit 17411ab)

appveyor.yml

@@ -94,13 +94,31 @@ deploy:
     artifact: /.*\.(msi|7z)/
     on:
         branch: snapshot
-        appveyor_repo_tag: false
         push_release: true
 
   - provider: Webhook
     url: https://app.signpath.io/API/v1/670574d6-49bf-4a0c-824a-3ae977077079/Integrations/AppVeyor?ProjectKey=tiled&SigningPolicyKey=test-signing
     authorization:
         secure: 4E7IuM1Ftvdkx43gsqI3tUWb6tcvrfKB22sa6DsWaBzT+zv3DqceDCk0qc/JqTO+Er5UQNYwolmWBzMdys6fVA==
+    on:
+        appveyor_repo_tag: false
+        push_release: true
+
+  - provider: Webhook
+    url: https://app.signpath.io/API/v1/670574d6-49bf-4a0c-824a-3ae977077079/Integrations/AppVeyor?ProjectKey=tiled&SigningPolicyKey=release-signing
+    authorization:
+        secure: 4E7IuM1Ftvdkx43gsqI3tUWb6tcvrfKB22sa6DsWaBzT+zv3DqceDCk0qc/JqTO+Er5UQNYwolmWBzMdys6fVA==
+    on:
+        branch: snapshot
+        push_release: true
+
+  - provider: Webhook
+    url: https://app.signpath.io/API/v1/670574d6-49bf-4a0c-824a-3ae977077079/Integrations/AppVeyor?ProjectKey=tiled&SigningPolicyKey=release-signing
+    authorization:
+        secure: 4E7IuM1Ftvdkx43gsqI3tUWb6tcvrfKB22sa6DsWaBzT+zv3DqceDCk0qc/JqTO+Er5UQNYwolmWBzMdys6fVA==
+    on:
+        appveyor_repo_tag: true
+        push_release: true
 
 notifications:
   - provider: Webhook