[META] phase 4

[oliverklee]

• [x] switch the URL to SemVer in the changelogs to HTTPS
• [x] drop the old core code
• [ ] #286 Cleanup: use Doctrine\ORM\Mapping as ORM;
• [ ] #287 use 32bit-safe timestamps as exception codes
• [x] upgrade phpstan
• [ ] add composer scripts for most of the Travis build steps
• [ ] use the directory names as defined in http://php-pds.com/
• [x] create a test branch with Symfony 4
• [ ] add other bundles that are standard for Symfony applications
• [ ] add the Symfony requirements checker
• [x] require Symfony >= 3.4 (and update the other dependencies)
• [x] make the project structure more similar to the default Symfony project structure
• [ ] research whether to use doctrine validations
• [ ] use Symfony bundle hooks
• [ ] research incenteenv
• [x] console command for purging old session tokens
• [ ] add functionality to automatically add new lines to parameters.yml
• [ ] decide on what to log
• [x] #2 add/configure logging
• [x] log things
• [ ] document the logging
• [ ] support/configure HTTP caching (last modified header, eTag, HEAD requests?), also research whether and how to use and set “valid until” headers
• [x] decide whether to switch to Symfony 4 (which would require PHP >= 7.1)
• [ ] phpList 3 Functionality dependency review (to determine order of next steps)
• [ ] #191 support SQLite
• [ ] create a "phplist4 module template" repository
• [ ] Include example module with composer package and repo, plus tips on adding DB tables, using hooks etc.
• [ ] provide a way to have different DB credentials in the tests via a yaml file (instead of via environment variables only)
• [ ] Configuration wizard that copies PL3 configuration to PL4 (and than can be run multiple times) (old configuration: https://resources.phplist.com/system/config/variables)