* Move action handling code to rcmail class
* Add rcmail_action class
* Add action aliases
* Get rid of $OUTPUT global
* Move some methods from rcmail to rcmail_action
* PHP8 compat. fixes
* Add framework for testing actions
* Fix obvious code mistakes
Fixes composer dependencies: Package phpunit/phpunit-mock-objects is abandoned
We cannot support v8 yet because of errors like:
Declaration of MailFunc::setUp() must be compatible with PHPUnit\Framework\TestCase::setUp(): void
It would require dropping PHP < 7.1 support.
