about_zulip: Migrate to handlebars.

This results in moving the `zulip_merge_base` parameter to
page_params, so that it's available to JavaScript.

Since this is technically a tiny overlay, it needs to be initialized
before hashchange.js.

zerver/tests/test_home.py

@@ -236,6 +236,7 @@ class HomeTest(ZulipTestCase):
         "webpack_public_path",
         "wildcard_mentions_notify",
         "zulip_feature_level",
+        "zulip_merge_base",
         "zulip_plan_is_not_limited",
         "zulip_version",
     ]