Silence errors about feature flags not existing

This fixes an issue where we get an error message about a feature flag
not existing when we try and create the feature flag in the factory.

It appears as though this is a bug or regression in Flipper itself, but
for now there is a workaround we can use to at least silence the error:
flippercloud/flipper#990

Author: thomasleese

app/lib/feature_flag_factory.rb

@@ -7,7 +7,9 @@ def self.call
         File.read(Rails.root.join("config/feature_flags.yml"))
       ).keys
 
-    names.each { Flipper.add(it) unless Flipper.exist?(it) }
+    Flipper::Adapters::Strict.with_sync_mode do
+      names.each { Flipper.add(it) unless Flipper.exist?(it) }
+    end
 
     Flipper.features.each do |feature|
       feature.remove unless feature.name.to_s.in?(names)