MDL-64505 theme_classic: Created the classic theme

The Classic theme is being introduced as a Bootstrap 4 replacement
for the older themes based on Bootstrap 2, such as Bootstrapbase,
Clean and More. It is a child theme of Boost, with a navigation
structure similar to the Clean theme.

Co-authored-by: Bas Brands <[email protected]>
Co-authored-by: Michael Hawkins <[email protected]>

lib/classes/plugin_manager.php

@@ -1905,7 +1905,7 @@ class core_plugin_manager {
             ),
 
             'theme' => array(
-                'boost', 'bootstrapbase', 'clean', 'more'
+                'boost', 'classic', 'bootstrapbase', 'clean', 'more'
             ),
 
             'tool' => array(