v0.4.1: Bump superagent from 3.5.2 to 3.7.0 in /webapp

Bumps [superagent](https://github.com/visionmedia/superagent) from 3.5.2 to 3.7.0.
- [Release notes](https://github.com/visionmedia/superagent/releases)
- [Changelog](https://github.com/visionmedia/superagent/blob/master/HISTORY.md)
- [Commits](https://github.com/visionmedia/superagent/commits/v3.7.0)

Deduplicate yarn.lock