* 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
- Only system extensions can be loaded for the moment by adding them in the config.php
file.
- Remove previous system (it will be added properly in the new system in the next step).