Improve error message when attempting to edit a deprecated version of a submission

[lognaturel]

From @dbemke in #794:

If a user tries to re-submit a submission which was edited in Central there is: “You tried to update a submission, but the copy you were editing (undefined) is now out of date. Please get the new version that has been submitted, and make your edits again.”

This message was designed specifically for the case where there's an attempt to edit a submission version that has been deprecated. This might happen, for example, if you open up two Enketo edit windows from Central, submit one, and then submit the other.

The case in the screenshot above is an attempt to submit the very first submission in an edit chain again after edits have been made. This is happening from Collect where there is no way to get the latest version so the second part of the message isn't helpful.

[lognaturel]

I think the most helpful thing for Central to do would be to accept the submission if the instanceid and submission contents exactly match an existing submission. This would be the same thing that happens if a submission without any following edits is resubmitted.