Move css units into container configuration

Author: zoglo

config/config.yaml

@@ -1,3 +1,6 @@
+contao_theme_manager_core:
+  config:
+    css_units: [ px, rem ]
 contao:
   image:
     sizes:

contao/config/config.php

@@ -16,6 +16,7 @@
 $GLOBALS['CTM_SETTINGS']['iconFont'] = '';
 
 // Register the supported CSS units for ThemeManager
+// @deprecated - to be removed in CTM 3
 $GLOBALS['CTM_CSS_UNITS'] = ['px', 'rem'];
 
 // Add configuration dca for themes