Fix poll votes not being properly reset on poll change (#17498)

* Fix poll votes not being properly reset on poll change

* Fix and add tests

* Fix poll update handling when the number of options changes

app/models/poll.rb

@@ -83,6 +83,12 @@ class Poll < ApplicationRecord
     end
   end
 
+  def reset_votes!
+    self.cached_tallies = options.map { 0 }
+    self.votes_count = 0
+    votes.delete_all unless new_record?
+  end
+
   private
 
   def prepare_cached_tallies