Add snippet url to webhook payload

All other type of note event payloads had a url in them
except for snippet.

Changelog: added

app/models/snippet.rb

@@ -242,7 +242,7 @@ def blobs(paths = [])
   end
 
   def hook_attrs
-    attributes
+    attributes.merge(url: Gitlab::UrlBuilder.build(self))
   end
 
   def file_name

doc/user/project/integrations/webhook_events.md

@@ -799,7 +799,8 @@ Payload example:
     "file_name": "test.rb",
     "expires_at": null,
     "type": "ProjectSnippet",
-    "visibility_level": 0
+    "visibility_level": 0,
+    "url": "http://example.com/gitlab-org/gitlab-test/snippets/53",
   }
 }
 ```

lib/gitlab/data_builder/note.rb

@@ -48,6 +48,7 @@ def build(note, user)
           data[:merge_request] = Gitlab::HookData::MergeRequestBuilder.new(note.noteable).build
         elsif note.for_snippet?
           data[:snippet] = note.noteable.hook_attrs
+          data[:snippet][:url] = Gitlab::UrlBuilder.build(note.noteable)
         end
 
         data