bot: Require +o to use !expire.

Some people seem to think that this is meant to be used frequently as a
way to get crawls on their own schedule.  They are wrong.

[ci skip]

bot/brain.rb

@@ -277,7 +277,7 @@ def add_ignore_sets(m, job, names, need_authorization=true)
   end
 
   def expire(m, job)
-    return unless authorized?(m)
+    return unless authorized?(m) && op?(m)
 
     if job.ttl < 0
       reply m, "Job #{job.ident} does not yet have an expiry timer."