Debug upgrade to Symf 3.4 + implement webpack encore

[leadermontanus]

• [x] Upgrade Symfony to 3.4 (LTS release, no LTS for 4.x yet)
• [x] install NPM/Nodejs https://nodejs.org/en/download/package-manager/
• [x] install Yarn (NPM alternative, used by Symfony for webpack-encore taks)
• [x] Redo from assetis to webpack config files.
• [x] Verify all JS is working OK
  • [x] Delete dialog box for components not working Caused by wrong order of jQuery file, mobile over-writes stuff in jquery ui!
• [ ] Split dev/prod in webpack
• [ ] Add uglify in webpack (alternatively use https://stackoverflow.com/questions/47183650/how-to-use-javascript-library-installed-with-npm-in-symfony)

Refs:

• https://symfony.com/doc/current/frontend.html
• https://symfony.com/doc/3.4/frontend/encore/installation-no-flex.html

Decided NOT to update Ubuntu, seems 18.4.1 is quite buggy for a lot of people still...

[leadermontanus]

To complete this we need to install an updated version of Encore (the Symfony wrapper for Webpack) which uses Webpack 4.x (current version of Encore supports versions 2 & 3).