Pico 3.0

[PhrozenByte]

Since the issues with our old dependencies kinda got out of hand lately (see https://github.com/picocms/Pico/issues/528, https://github.com/picocms/Pico/issues/534 and picocms/Pico@8a44584), it's time to kick-start development of Pico 3.0 :tada:

Major changes are going to be updated dependencies (Twig 3.0+ and Symfony YAML 5.0+) and lifting the requirements to PHP 7.2.5+. We […] [will] focus on lazy page discovery and loading.

As always, feedback is highly appreciated! :heart:

Please use this PR for code reviews only, for a list of major changes and for comments please refer to https://github.com/picocms/Pico/pull/535