Upgrade to Slim 4

[Findus23]

Third time's a charm :)

This is a recreation of #213 using the latest version of Slim 4 that again changed quite a lot of things.

This only includes the most important changes to get Slim 4 working, any other improvement to the site will come in later pull requests to keep this small.

• updated Slim to 4
• updated Twig to 3.x
• use latest michelf/php-markdown
• requires at least PHP 7.2.0
• also cache json routes
• use proper file extensions for cache files
• don't cache if page contains failed request
• cache routing to tmp/cache/route_cache.php
• completely rewritten app/public/index.php as it contains the slim-specific logic
• lots of reformatting in app/routes/page.php to have routes in new format
• partly revert Remove docs for Piwik 2 #277 (show search again in navbar)
• minor change in app/public/js/app.js to make search work again.

I'm still totally confused by Dependeny Injection so this is written in a "normal" style instead of as recommended in https://github.com/slimphp/Slim-Skeleton/blob/master/public/index.php.

A demo page running this branch can be found at https://matomodocs.lw1.at/

[tsteur]

👍