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/topic.rb

@@ -15,5 +15,4 @@ class Topic < ApplicationRecord
   scope :sort_by_newest, -> { order(created_at: :desc) }
   scope :sort_by_oldest, -> { order(created_at: :asc) }
   scope :sort_by_most_commented, -> { reorder(comments_count: :desc) }
-
 end