PageImages is expensively loading and reparsing the lead section during
LinksUpdate because the parser's image link hooks do not give enough
context to tell whether the image is in the lead section.
So, add a hook which allows PageImages to add a marker to image links.
The marker can be associated with the section number in ParserAfterTidy.
Bug: T296895
Bug: T176520
Change-Id: I24528381e8d24ca8d138bceadb9397c83fd31356