WikiTextStructure: Also extract figcaption elements as captions

The figcaption element looks to be many years old, but was not
previously considered a caption by the search engine. Some integration
tests have recently started failing as an element that previously
contained the thumbcaption css element is now inside a figcaption
class.

Update the code that extracts captions from the text into a separate
field to also extract the figcaption, as it has the same purpose as
thumbcaption.

Change-Id: I2a4a309e58602281d6cca65744036efb4a5ce5b5

includes/content/WikiTextStructure.php

@@ -52,6 +52,7 @@ class WikiTextStructure {
 	private $auxiliaryElementSelectors = [
 		// Thumbnail captions aren't really part of the text proper
 		'.thumbcaption',
+		'figcaption',
 		// Neither are tables
 		'table',
 		// Common style for "See also:".