BURs: fix mass update A -> B not being allowed.

Fix mass updates of the form `mass update A -> B` not being allowed.

This was originally because after `rename` was introduced, we wanted to
prevent people from using mass updates to move tags. Now, `mass update A -> B`
adds B to all posts tagged A instead of moving A to B. So `mass update A -> B`
should no longer be disallowed.

This also makes it so that it's an error to create a mass update with a
syntax error in the search. Before searches couldn't have syntax errors,
but now with the new query parser it's possible.

app/logical/post_query.rb

@@ -16,6 +16,16 @@ class PostQuery
     Tag.where(name: tag_names)
   end
 
+  # True if this search would return all posts (normally because the search is the empty string).
+  def is_empty_search?
+    ast.all?
+  end
+
+  # True if this search would return nothing (normally because there was a syntax error).
+  def is_null_search?
+    ast.none?
+  end
+
   def is_single_tag?
     ast.tag?
   end