MDL-25012 - blogs: introduce a specific 'enabled' config setting

Rather than overloading the $CFG->bloglevel setting which had a
confusing UI in the appearance subsystem.

In order to achieve this we modify take the defaults from the existing
bloglevel setting and set that for $CFG->enableblogs. Note that in order
to prevent a bad default settings from being set we also set
$CFG->bloglevel to a valid 'enabled' setting.

lib/filelib.php

@@ -3451,7 +3451,7 @@ function file_pluginfile($relativepath, $forcedownload, $preview = null) {
             send_file_not_found();
         }
 
-        if (empty($CFG->bloglevel)) {
+        if (empty($CFG->enableblogs)) {
             print_error('siteblogdisable', 'blog');
         }