text: Title: Concurrent editing notifications - backend additions to identify concurrent edits
  Body: Part of [RFC 95 - concurrent editing notifications](https://github.com/wagtail/rfcs/pull/95)
  
  As per the RFC section 'Notifying users on changes', add support to the 'ping' endpoint for passing a revision ID so that any users who have created revisions newer than that one are annotated as part of the response, including users who do not have an active editing session open.
  
  (TBC: does this backend code need to know about the list of dismissed revision IDs, or is this entirely a front-end detail? If the former, that should be covered as part of this issue.)
  
  ### Working on this
  
  To be implemented by the Wagtail product team at Torchbox - we are not seeking external contributors for this.