Display a banner if there's a change request for a page

Right now it's easy to miss that there's a CR for a page and to start editing it, or worse, to rename it. A banner at the top of the page would help notice it.

The doc app does something similar, see step 4 of https://www.xwiki.org/xwiki/bin/view/Doc/extensions/user/documentation/create-documentation-page/

Note that ideally we would also have some warning in the rename UI if we rename a page wit a CR since that will break the CR (the CR won't know that the page has been renamed). A better option is ofc to have the CR app notice that the page has been renamed by listening to rename events and update the CRs accordingly.