Add and apply rubocop rules for empty lines

We were very inconsistent regarding these rules.

Personally I prefer no empty lines around blocks, clases, etc... as
recommended by the Ruby style guide [1], and they're the default values
in rubocop, so those are the settings I'm applying.

The exception is the `private` access modifier, since we were leaving
empty lines around it most of the time. That's the default rubocop rule
as well. Personally I don't have a strong preference about this one.


[1] https://rubystyle.guide/#empty-lines-around-bodies

app/models/poll/stats.rb

@@ -120,5 +120,4 @@ class Poll::Stats
     def stats_cache(key, &block)
       Rails.cache.fetch("polls_stats/#{poll.id}/#{key}/#{version}", &block)
     end
-
 end