Allow experiment FFs to be in CE or EE

- Adjust the definition of `experiment` feature flag type to allow for
  YAML files to be in both config/... and ee/config/...
- Review usage of existing `experiment` type feature flags in the code &
  move the corresponding YAML file from ee/config/... to config/... as
  necessary

lib/feature/shared.rb

@@ -53,7 +53,7 @@ class Feature
         description: 'Short lived, used specifically to run A/B/n experiments.',
         optional: true,
         rollout_issue: true,
-        ee_only: true,
+        ee_only: false,
         default_enabled: false,
         example: <<-EOS
           experiment(:my_experiment, project: project, actor: current_user) { ...variant code... }