Add `is_ratelimited` helper function

So we have a case on SUMO where we need to limit the number of new questions that a user can post. Posting a new question has several steps which all happen in the same view function. There are also things like form validation which happen which make using `skip_if` impossible. The decorator is not able to deal with the complexity of ratelimiting this view. 

Hence, helper function!

r?