Actually detect duplicate section=new submissions

The current check will basically never pass, since it's looking for the
submitted section title to match the section title inserted into the
'newsectionsummary' message.

So let's factor out the code for applying 'newsectionsummary' and use
that for the conflict check. We can't just update $this->summary
earlier, since later checks depend on looking at the submitted summary
rather than the mangled one.

Bug: 67634
Change-Id: I72890c0641f991696ec98b50a6a42b2be7f46f63
Merge "Actually detect duplicate section=new submissions"