The autosave doesn't behave nicely when a merge conflict occurs

This is similar to XWIKI-19180 but instead of clicking on the Save button Bob and Carol (from the real-time session) wait for the auto-save to happen. So the problem is when the auto-save fails because there's a merge conflict due to changes outside of the real-time session. There are multiple problems:

• Bob and Carol get a modal (non-bootstrap) explaining the save error but there's no much they can do. Instead they should be the standard Merge Conflict modal that we see in XWIKI-19180 when clicking on the save button.
• If Bob and Carol wait a bit the modal becomes blank. In the JavaScript console I can see the JavaScript code is retrying to save and fails again and again.