Bump eventsource, sockjs-client and webpack-dev-server (#1026)

Bumps [eventsource](https://github.com/EventSource/eventsource), [sockjs-client](https://github.com/sockjs/sockjs-client) and [webpack-dev-server](https://github.com/webpack/webpack-dev-server). These dependencies needed to be updated together.

Updates `eventsource` from 0.1.6 to 2.0.2
- [Release notes](https://github.com/EventSource/eventsource/releases)
- [Changelog](https://github.com/EventSource/eventsource/blob/master/HISTORY.md)
- [Commits](EventSource/eventsource@v0.1.6...v2.0.2)

Updates `sockjs-client` from 1.1.4 to 1.6.1
- [Release notes](https://github.com/sockjs/sockjs-client/releases)
- [Changelog](https://github.com/sockjs/sockjs-client/blob/main/Changelog.md)
- [Commits](sockjs/sockjs-client@v1.1.4...v1.6.1)

Updates `webpack-dev-server` from 2.3.0 to 4.11.0
- [Release notes](https://github.com/webpack/webpack-dev-server/releases)
- [Changelog](https://github.com/webpack/webpack-dev-server/blob/master/CHANGELOG.md)
- [Commits](webpack/webpack-dev-server@v2.3.0...v4.11.0)

---
updated-dependencies:
- dependency-name: eventsource
  dependency-type: indirect
- dependency-name: sockjs-client
  dependency-type: direct:production
- dependency-name: webpack-dev-server
  dependency-type: direct:development
...

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