[META] phase 4

[oliverklee]

• switch the URL to SemVer in the changelogs to HTTPS
• drop the old core code
• [CLEANUP] Reduce the number of Doctrine-related imports #286 Cleanup: use Doctrine\ORM\Mapping as ORM;
• [BUGFIX] Make the exception codes 32-bit safe #287 use 32bit-safe timestamps as exception codes
• upgrade phpstan
• add composer scripts for most of the Travis build steps
• use the directory names as defined in http://php-pds.com/
• create a test branch with Symfony 4
• add other bundles that are standard for Symfony applications
• add the Symfony requirements checker
• require Symfony >= 3.4 (and update the other dependencies)
• make the project structure more similar to the default Symfony project structure
• research whether to use doctrine validations
• use Symfony bundle hooks
• research incenteenv
• console command for purging old session tokens
• add functionality to automatically add new lines to parameters.yml
• decide on what to log
• PSR-3 logging to replace output #2 add/configure logging
• 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
• decide whether to switch to Symfony 4 (which would require PHP >= 7.1)
• phpList 3 Functionality dependency review (to determine order of next steps)
• Support SQLite #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)