Search: decouple tags and questions and include answers in results

[cellio]

https://meta.codidact.com/posts/288176

If you search for answers (post type 2) and the name of a tag, you get no results. That's because tags are associated with questions.

If you search for some phrase and a tag, not specifying post types, you get only top-level posts, the ones that can have tags. But sometimes the search terms you remember came from an an answer and you'd like to find it.

If, for purposes of search, we applied a question's tags to answers too, then I think both of these would work.

Implementation note: we have sometimes talked about letting answers have tags of their own, so that, for example, a question tagged python could have answers tagged python2 and python3. That would be a separate feature requiring more investigation. I'm mentioning it here so we avoid doing something that would cut off that path later. A possible implementation of this feature request would be to add a "tags" column for answers and sync the values with the question, which might collide with "real" answer tags in the future. So if we do it that way, name the column something else like "inherited_tags", please.

[cellio]

Alternatively, no DDL change and, for post types that have parents, search gets the tags from the parent. I don't know if the extra check at runtime is better or worse than the db change (to add inherited tags to child posts).