Fix #5132: Modqueue displays active posts when excluding any search term

Fix a bug where searching for a negated tag inside the modqueue would show
active posts.

The bug was that in a search like this:

    Post.in_modqueue.user_tag_match("-solo")

The `in_modqueue` condition would get sucked inside the tag search and negated
when we tried to apply the negation operator to the "solo" tag. So effectively
the `in_modqueue` condition would get negated and we would end up searching for
everything not in the modqueue.

app/logical/post_query_builder.rb

@@ -255,8 +255,7 @@ class PostQueryBuilder
     end
   end
 
-  def posts(post_query, includes: nil)
-    relation = Post.all
+  def posts(post_query, relation = Post.unscoped, includes: nil)
     relation = add_joins(post_query, relation)
     relation = build_relation(post_query, relation)