Fix display of search tag results without holder information

JDK-8253559 adds several new items to the search index which require changes in the search script. Specifically, they do not contain a `holder` property which is currently expected from search tag results.

Other issues to clean up or improve:
 - SearchTags is written as a single word in search results
 - The term "search tags" refers to an authoring concept, maybe we can find a better (more reader centric) term?