Discards user work when multiple users are editing

I have experienced a number of instances where the Numbas editor has lost work that I did: changes I have made to question statements and to grading settings have disappeared overnight. I *think* that the problem is that my teaching assistant was also working on the same question, or at least had the question open in a tab.

My guess as to the process:

- Person A opens the question/part, loading the content.
- Person B opens the question/part, loading the content.
- Person A makes some change, and it is automatically saved.
- Person B does not see the change, and makes some other unrelated change. The auto-save reverts person A's change.

There is an alternative way to lose changes described in #731 that can happen if you leave pages too soon; this happens if you leave pages open too long.

Start by reproducing the reported sequence with two users editing the same question or part, including automatic saves and leaving pages open. Trace the question/part editor's auto-save flow and compare each user's loaded content with the saved result. Done means one user's unrelated changes no longer discard another user's work, with the behavior covered by an appropriate test.

Written by the indexing model from the issue text.