Only normalize local polls (#12515)

Before this patch, if remote poll options have leading or trailing spaces,
the information stored locally won't match them, causing federated voting to
fail.

app/models/poll.rb

@@ -36,7 +36,7 @@ class Poll < ApplicationRecord
   scope :attached, -> { where.not(status_id: nil) }
   scope :unattached, -> { where(status_id: nil) }
 
-  before_validation :prepare_options
+  before_validation :prepare_options, if: :local?
   before_validation :prepare_votes_count
 
   after_initialize :prepare_cached_tallies