Initial OpenCATS running on Symfony + Codeception test framework

[mlespiau]

Wiki draft: https://github.com/opencats/OpenCATS/wiki/Dev-Guide-to-migrate-Legacy-to-Symfony Changes:

• Add symfony project to github repo
• Ignore symfony dynamic files in .gitignore
• Create a Symfony Controller (LegacyController) to bridge OpenCATS and Symfony. Now all requests go through Symfony and are routed to "legacy" OpenCATS code.
• Enabled opcache in docker image (required for Symfony performance)
• Use legacy session management in symfony to make it compatible with "legacy" OpenCATS
• Move all static assets (css, js, images) to web folder
• Add toolbar UI support for OpenCATS chrome extension
• Replace whole testing tooling with Codeception and updated all test code to work with it
• Use a new simple docker image with php + nginx + application code

[skrchnavy]

@mlespiau This PR shall have dependency on #257 and #260, when those PRs merged to develop, then develop shall be merged to this PR's branch (conflict solved using mine - to use symfony branch code)

Then we can concentrate on review of real changes.