Improve warning conflict to show diff and force save or reload editor to latest version

Starting with XWiki 11.2RC1 we can get a version conflict when saving a page, see XWIKI-6665 . I think we need to have a way to force the save because:

• the conflict might not be real (a bug like XWIKI-16343)
• it may be easier to force the save and roll back afterwards some of the changes lost than to copy the changes made by the current user. Take for instance this use case:
  • user A starts editing and makes a lot of changes in various parts of the content
  • user B edits the same page to fix a typo
  • user A wants to save and gets the version conflict. He checks the diff. It would certainly be easier for him to force the save and fix the typo afterwards than to copy all the various changes he did.