Move XWiki Syntax Help panel into Help Application

Raised as a consequence of a discussion in the XWiki Forum: https://forum.xwiki.org/t/is-it-safe-to-uninstall-the-help-application/10499/3

If the Help Application is uninstalled, a broken link for Syntax Help may still appear when editing a page. As such, the "XWiki Syntax Help" panel would need to be removed separately. It's also a panel that is displayed by default (see here)

Ideally, the panel would be moved into the Help Application with a means for the Help App to inject the panel into the list of default edit panels.