Previously, `$this->summary` was used for two different purposes.
Usually it was just the summary. But when `$this->section` was 'new',
then it was actually the section title most of the time – unless
`$this->sectiontitle` was also set (in which case it took priority),
and until it was replaced by the real edit summary (near the end of
the processing, after copying the section title to the page content
and before saving changes).
Unsurprisingly some of the code didn't handle this duality correctly,
causing T191722 and T311533.
Now `$this->summary` is always the summary, and when `$this->section`
is 'new', then `$this->sectiontitle` is always the new section title.
The only place where this duality remains is in the input attributes
and query parameters, where 'wpSummary' is still used for both the
summary and the section title inputs (only one of them can appear,
depending on whether `$this->section` is 'new'). It would be an
unreasonable backwards-compatibility break to change this, and the
code handling this is somewhat isolated from the rest of the logic.
Bug: T191722
Bug: T311533
Change-Id: I5313ca9a045d112ece390b011a34192220e2abc1
990fd8f0b2