Restructured FluxCP to be PSR-4 compliance

[secretdataz]

• Now using Composer to handle dependencies. Supersedes Composer-ing components #210.
• Now using PSR-4 autoloader to handle class loading. No more requires_once hell.
• Also used filp/whoops to handle errors and exceptions instead of Flux's own handler.
• FluxCP no longer expose unnecessary information in production environment unless configured to.
• Moved some environment-specific settings into .env file. Provides more flexible developer experience.
• Breaks every single existing themes and addons. Updates required are minor since modules and themes haven't been restructured.

[sanasol]

That's awesome no doubt.
But killing all themes/addons and backward compatibility is looks really bad.

Maybe we should try to do softer upgrade first and keep compatibility at least for some fixed time.
So addon/theme makers will have time to upgrade their code.

Also need prepare step-by-step upgrade guide.
I believe that laravel-style will perfectly fit for it - https://laravel.com/docs/5.8/upgrade

[secretdataz]

According to Cydh and Akkarin's suggestion, they are okay with setting a life time for current FluxCP version and draft a major release. While we wait for the end-of-life, we can add more things to the soon-to-be major release.

[secretdataz]

Superseded by #233