[Feature Flags] Give custom DC their own checked in flags. (#4934)

Creates a local file with the feature flags to be used in Custom DC.
Sets autocomplete and place page flags to be disabled (note that the
screenshot had place experiment enabled, but it's now disabled).

Starts reading the flags for custom DC from the local file, and injects
them in the custom DC base.html file.

See how local custom DC now has the scroll to top button:
https://screenshot.googleplex.com/7dk4eim4LALCuVL

server/config/feature_flag_configs/custom.json

@@ -0,0 +1,24 @@
+[{
+    "name": "autocomplete",
+    "enabled": false,
+    "owner": "gmechali",
+    "description": "Feature flag to enable place autocomplete."
+},
+{
+    "name": "dev_place_experiment",
+    "enabled": false,
+    "owner": "gmechali",
+    "description": "Feature flag to enable the V2 place page only for the experiment places."
+},
+{
+    "name": "dev_place_ga",
+    "enabled": false,
+    "owner": "dwnoble",
+    "description": "Feature flag to enable the V2 Place page to GA."
+},
+{
+    "name": "scroll_to_top_button",
+    "enabled": true,
+    "owner": "gmechali",
+    "description": "Feature flag to enable the scroll to top button on the explore and place pages."
+}]

server/lib/util.py

@@ -431,7 +431,7 @@ def load_feature_flags_from_gcs(environment: str):
 def load_fallback_feature_flags(environment: str):
   """Loads the fallback feature flags into the app config. We fallback to checked in flag configs per environment."""
   environments_with_local_files = set(
-      ['local', 'autopush', 'dev', 'staging', 'production'])
+      ['local', 'autopush', 'dev', 'staging', 'production', 'custom'])
   testing_environments = set(['integration_test', 'test', 'webdriver'])
 
   if environment in testing_environments:

server/templates/custom_dc/custom/base.html

@@ -49,6 +49,9 @@
     });
   </script>
   {% endif %}
+  <script>
+    globalThis.FEATURE_FLAGS = {{ config['FEATURE_FLAGS'] | tojson }};
+  </script>
 
   <title>{{ title }} - Custom Data Commons</title>
   <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />