MDL-56488 boost: validate scss settings for parsable scss

* Parses the scss and fails validation if a parse error is found

* Does not detect the situation when variables are not present - this
  would involve us parsing the entire tree and would be slow. It could
  also change over time, depending on whats defined in the scss
  from themes.

* Introduces a new admin_setting_scsscode to do this

lib/adminlib.php

@@ -10180,3 +10180,41 @@ class admin_setting_searchsetupinfo extends admin_setting {
     }
 
 }
+
+/**
+ * Used to validate the contents of SCSS code and ensuring they are parsable.
+ *
+ * It does not attempt to detect undefined SCSS variables because it is designed
+ * to be used without knowledge of other config/scss included.
+ *
+ * @license http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ * @copyright 2016 Dan Poltawski <dan@moodle.com>
+ */
+class admin_setting_scsscode extends admin_setting_configtextarea {
+
+    /**
+     * Validate the contents of the SCSS to ensure its parsable. Does not
+     * attempt to detect undefined scss variables.
+     *
+     * @param string $data The scss code from text field.
+     * @return mixed bool true for success or string:error on failure.
+     */
+    public function validate($data) {
+        if (empty($data)) {
+            return true;
+        }
+
+        $scss = new core_scss();
+        try {
+            $scss->compile($data);
+        } catch (Leafo\ScssPhp\Exception\ParserException $e) {
+            return get_string('scssinvalid', 'admin', $e->getMessage());
+        } catch (Leafo\ScssPhp\Exception\CompilerException $e) {
+            // Silently ignore this - it could be a scss variable defined from somewhere
+            // else which we are not examining here.
+            return true;
+        }
+
+        return true;
+    }
+}