MDL-53695 minify: switch to MatthiasMullie\Minify

The previously used minification libraries were:
a) Unmaintained
b) Had some silly licensing issues (MDL-36457)

The new library is smaller, maintained, active and GPL-compatible.

lib/classes/minify.php

@@ -35,18 +35,9 @@ class core_minify {
      * @return string minified JS code
      */
     public static function js($content) {
-        global $CFG;
-        require_once("$CFG->libdir/minify/lib/JSMinPlus.php");
-
         try {
-            ob_start(); // JSMinPlus just echos errors, weird...
-            $compressed = JSMinPlus::minify($content);
-            if ($compressed !== false) {
-                ob_end_clean();
-                return $compressed;
-            }
-            $error = ob_get_clean();
-
+            $minifier = new MatthiasMullie\Minify\JS($content);
+            return $minifier->minify();
         } catch (Exception $e) {
             ob_end_clean();
             $error = $e->getMessage();
@@ -97,16 +88,10 @@ EOD;
      * @return string minified CSS
      */
     public static function css($content) {
-        global $CFG;
-        require_once("$CFG->libdir/minify/lib/Minify/CSS/Compressor.php");
-
         $error = 'unknown';
         try {
-            $compressed = Minify_CSS_Compressor::process($content);
-            if ($compressed !== false) {
-                return $compressed;
-            }
-
+            $minifier = new MatthiasMullie\Minify\CSS($content);
+            return $minifier->minify();
         } catch (Exception $e) {
             $error = $e->getMessage();
         }