Allow a message with more than one event to be destroyed.

Fixes #1569

@garethrees could you take a look, and do you want to add a fix for #1563 before we roll it out as a hotfix? 
Might be able to use [`destroy_all`](http://api.rubyonrails.org/v3.2.18/classes/ActiveRecord/Relation.html#method-i-destroy_all) here which I assume has better performance.