Increase choices size for polls

MariaDB/MySQL stores the array as serialized string, so the current
limit of 255 is too small to store all choices.

db/migrate/20181120000000_increase_choices_size.foodsoft_polls_engine.rb

@@ -0,0 +1,5 @@
+class IncreaseChoicesSize < ActiveRecord::Migration
+  def up
+    change_column :polls, :choices, :text, limit: 65535
+  end
+end