Update Symfony to 4.x

[sproogen]

Symfony 4 is now a thing, look at updating.

• [x] Composer Update
• [x] Move everything from ItsGoingToBeBundle
• [x] Autowire & Autoconfigure
• [x] Re-add PDO Session Handler
• [x] Tests
• [x] React client
  • [x] Move location
  • [x] Update build scripts
• [x] Environment variables
  • [x] Travis CI
  • [x] Docker
  • [x] RSA password in composer
  • [x] Production ENVs

[sproogen]

https://symfony.com/doc/current/setup/upgrade_major.html

[sproogen]

https://knpuniversity.com/screencast/symfony4-upgrade/framework-finish#play