Uncomment and feature toggle sets ui

Summary:
Sets UI code its partially wired and but commented out in the UI.
Instead we can just hide it with a feature toggle

Test Plan: - Enable 'sets_ui' feature and sets appear in header and add-to-set links on posts

Reviewers: shadowhand

Reviewed By: shadowhand

Differential Revision: https://phabricator.ushahidi.com/D77