Fixes #3147 Add rel="noreferrer" to all outgoing links . This works in Firefox so far and only for "left clicks". Not perfect, but hopefully other browsers will implement this in the future, as it's a useful privacy enhancing feature!

refs #5940 moved files into a subfolder framework, added autoloader to remove duplicated code to load autoload.php and to be able to register more autoloaders (eg for test files) on demand. This I got read of many includes that had to be updated all the time and that had to be updated all the time when moving iles

- renamed code coverage tests units to new structure
- removed processed system test files from git, added to .gitignore

refs #5940 we might stay backwards compatible this way for 2 months. moved some tests from unit to integration and explained when a test is a unit test, an integration test or a system test.

refs #5940 put tests in correct folders, better testsuite names, some tests still fail and I cannot figure out why

Fixes #6313 - When a user starts to get a User ID set, then we set this User ID to the existing visit rather than create a new visit for this user id specifically

coding style fixes, some PHPStorm inspection fixes, improved readability of code, few refactorings, all as part of our code cleanup strategy

User ID is NULLable now results in expected tests results: nb_users = 0 when no user id was set. refs #3490 #6162

refs #6162 when aggregating periods, do not sum 'Unique Users' metric as this is not a valid operation.