DO NOT merge/review - EZP-21846: Contact form using symfony

Link: https://jira.ez.no/browse/EZP-21846

## Description
Information collectors have not been implemented on 5.x. So for now, the DemoBundle displays a way to build a contact form using the Symfony form and to send an email using swiftmailer.

## Tests
Manual tests

## TODO
- [x] Form controller
- [ ] General cleanup
- [ ] Finish html/css
- [ ] Rebase commits