Add automated test for "Reply To Annotation"

The manual test is here: Reply To Annotation

Existing automated coverage

• AnnotationCodeMacrosPageTest#replyButtonFallsBackWhenXRedirectIsMissing: Unit level: the annotation bubble renders a Reply button whose URL targets the annotated page with replyto=<comment id> and the comment anchor (also replyButtonRejectsJavascriptXRedirect, replyButtonKeepsXRedirect)

Missing automated coverage

• With annotations shown, click a highlighted annotation and click the 'Reply' (chat bubble) button in the annotation bubble
• Write a reply and click 'Add Comment'; assert the reply is saved as a reply (replyto) of the annotation comment in the Comments tab
• Click the highlighted annotated text again and click the 'View thread' link (annotations.action.view.thread)
• Assert the Comments tab is opened and the annotation comment thread (with the reply) is highlighted/scrolled to
• Add the needed page-object methods on AnnotationsLabel (e.g. clickReply, clickViewThread)

Suggested location: xwiki-platform-core/xwiki-platform-annotation/xwiki-platform-annotation-test/xwiki-platform-annotation-test-docker/src/test/it/org/xwiki/annotation/test/ui/AnnotationsIT.java

Found by comparing the XWiki 18.4.4 manual test runs against the automated tests on master (a0055f10a1a).