While rewriting the Cite extension, we tiptoed around two edge cases for the `follow` feature (see [[ https://www.mediawiki.org/wiki/Help:Cite#Merging_two_texts_into_a_single_reference | manual ]]). This is something used on Wikisource when splitting long footnotes broken across pages in the published document. The PHP implementation of that feature causes considerable technical debt in Cite, this task's scope is to cleaned up most of the debt, or documented a path to fixing in the future. Don't change behavior if possible, but if there are changes please announce them as part of this task.
Existing logic:
* In normal usage, `<ref name="first">First text</ref>` will precede `<ref follow="first">Second text</ref>`, producing a footnote marker for only the first reference, and the text "First text Second text" in the references section.
* If used incorrectly, with the two refs above swapped so that `follows` precedes the "First text" ref, we will not show a footnote mark in the article body, but will show the follow ref's text at the top of the references section, without any error indicators. This is different than how any other erroring ref is handled.
* There are a ton of smaller edge cases, which may or may not be worth listing here.