[bug 1233001] Add client.channel in cache key.

Since snippets_js changes based on client channel we need to add that
too in the cache key generation recipe. Once we complete MLS migration
and release switches too we can safely delete this code.

snippets/base/models.py

@@ -139,6 +139,7 @@ def key(self):
         key_properties.extend([
             self.client.startpage_version,
             self.client.locale,
+            self.client.channel,
             SNIPPET_JS_TEMPLATE_HASH,
             SNIPPET_CSS_TEMPLATE_HASH,
             SNIPPET_FETCH_TEMPLATE_HASH,