Change decision letter parsing for eLife articles.

[gnott]

Re issue https://github.com/elifesciences/issues/issues/6463

A short summary of the bug to be fixed in this PR, eLife Decision letter and Author response <sub-article> peer review articles previously had @id attribute values of SA1 and SA2, and these were used in the query selectors in Lens' elife_converter.js to find them.

In more recent eLife articles the @id values have gone lowercase, sa1 and sa2, which causes them to not display on Lens.

Initially we were planning to change the logic to look at the @article-type of the <sub-article> tag to determine whether the node should be assigned a heading of "Decision letter" or "Author response". After some consideration of how the <sub-article> tags could be processed in a loop, it looked to be easier to take the Lens node heading value from the <article-title> tag of the <sub-article>.

This should only affect eLife articles since we are only editing the converter/elife_converter.js file. The logic here should also be backwards compatible for all eLife articles based on the previously published article data I checked. It also does not cause any errors if there are no <sub-article> tags in the article XML.