Symfony 4.x migration

[luispabon]

• Upgrade current set up to use Symfony 4.x.
• Keep using assetic for now - a react rewrite of the UI will be coming soon and there's no need to go down the webpack-encore rabbit hole on the slapdash JS code we have on the site

[luispabon]

Actually, scratch this - Symfony 3.4 is LTS until 2021 so no need to create work for ourselves here.