* New extension event + Tumblr GDPR
https://github.com/FreshRSS/FreshRSS/issues/1894
simplepie_before_init event
* Refactor extension enabling + Tumblr GDPR enabled by default
Add possibility for extensions to be enabled by default, and disabled
back by users.
* Minor whitespace
- 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).