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/bulk_update_request_processor.rb

@@ -116,11 +116,10 @@ class BulkUpdateRequestProcessor
           end
 
         when :mass_update
-          lhs = PostQuery.new(args[0])
-          rhs = PostQuery.new(args[1])
+          query = PostQuery.new(args[0])
 
-          if lhs.is_single_tag? && rhs.is_single_tag?
-            errors.add(:base, "Can't mass update #{args[0]} -> #{args[1]} (use an alias or a rename instead for tag moves)")
+          if query.is_null_search?
+            errors.add(:base, "Can't mass update #{args[0]} -> #{args[1]} (the search `#{args[0]}` has a syntax error)")
           end
 
         when :nuke