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.
fb0a7851bf