The output of CommentFormatter::addReplyLinks() and consequently
ThreadItem::jsonSerialize() can end up in the HTTP cache (Varnish) on
Wikimedia wikis. We need to consider that when changing that code.
Introduce a concept of legacy ID (generated by the older algorithm
after it changes), add some placeholder code that will generate them
in the future, and update some code to find comments by either normal
or legacy IDs.
Add dire comments in a bunch of places (as if that ever helps).
Bug: T264478
Change-Id: I4368f366800ab21b8b184b09378037614fdecd33
