* Fix extension freshrss_user_maintenance in actualize_script
Follow-up of https://github.com/FreshRSS/FreshRSS/pull/3440
The hook was called before registering all the extensions for the current user
* PHPStan Level 6 for extensions
And remove 5-year old legacy format of enabled extensions < FreshRSS 1.11.1
* Fix multiple bugs in extensions
* Minor typing
* Don't change signature of methods supposed to be overridden
* PHPStan Level 9 and compatibility Intelliphense
* Set as final the methods not supposed to be overriden
* New core-extensions to allow Docker volumes for third-party extensions
#Fix https://github.com/FreshRSS/FreshRSS/issues/2650
Split our extensions directory into two:
1) Core extensions shipped with FreshRSS in ./lib/core-extensions/
2) Third-party extensions modified by end-users in ./extensions/ which
can easily be mounted as a Docker volume
* Example of Docker Compose with extensions
* Back-compatibility + fix array merge bug
Alexandre Alapetite