[3.0] Move external libraries to a separate directory (Libs)

[dragomano]

This PR moves third-party libraries (Minify and ReCaptcha) to a separate subdirectory within Sources — Libs.

It also adds a command (see composer.json in the root) to update these libraries (experts can to configure this command to run before building the package so that the Libs folder is included in the SMF distribution package).

Also implemented are 2 of SMF's own classes, JS and CSS, which extend the external classes MatthiasMullie\Minify\JS and MatthiasMullie\Minify\CSS respectively. This is done to add an additional comment at the top of each minimized file (previously this required changes to the updated Minify.php each time).

This way, external libraries can be easily updated with each release.

Fixes #7977

[live627]

Also implemented are 2 of SMF's own classes, JS and CSS, which extend the external classes MatthiasMullie\Minify\JS and MatthiasMullie\Minify\CSS respectively. This is done to add an additional comment at the top of each minimized file (previously this required changes to the updated Minify.php each time).

• [3.0] Manage third party libs via composer #8240 does not add this.