Add global CSS to hide core maintenance warning

- The core maintenance mode message is redundant and appears
      in some circumstances that are undesirable - e.g. inside
      active SCORM activities or prior to the defined warning
      message period.

styles.css

@@ -0,0 +1,8 @@
+/**
+ * Globally supress core Moodle maintenance warning message.
+ *
+ * Note that this can be explicitly overridden by themes.
+ */
+.box.moodle-has-zindex.maintenancewarning {
+    display: none;
+}