feat: do not report Media Overlays ordering mismatch

EPUBCheck v4.2.5 introduced a new check (MED-015) to report a mismatch
between Media Overlays and Content Document order.
But this rule will be relaxed in EPUB 3.3.

This commit preemptively demotes MED-015 to a USAGE.

See #1225. We'll decide if we want to totally remove the feature when we
fully implementat EPUB 3.3.

Author: rdeltour

src/main/java/com/adobe/epubcheck/messages/DefaultSeverities.java

@@ -151,7 +151,7 @@ private void initialize()
     severities.put(MessageId.MED_012, Severity.ERROR);
     severities.put(MessageId.MED_013, Severity.ERROR);
     severities.put(MessageId.MED_014, Severity.ERROR);
-    severities.put(MessageId.MED_015, Severity.ERROR);
+    severities.put(MessageId.MED_015, Severity.USAGE);
 
     // NAV
     severities.put(MessageId.NAV_001, Severity.ERROR);

src/test/resources/epub3/mediaoverlays-publication.feature

@@ -50,8 +50,9 @@ Feature: EPUB 3 ▸ Media Overlays ▸ Full Publication Checks
   ### 3.2.1 Structure
 
   Scenario: Report an overlay document whose text elements do not match the dom order of the corresponding content document
+    Given the reporting level is set to USAGE
     When checking EPUB 'mediaoverlays-text-reading-order-error'
-    Then error MED-015 is reported
+    Then usage MED-015 is reported
     And no other errors or warnings are reported
 
   ### 3.5.1 Including Media Overlays