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

spec/models/topic_spec.rb

@@ -27,7 +27,6 @@ describe Topic do
   end
 
   context "order" do
-
     it "orders by newest" do
       proposal = create(:proposal)
       community = proposal.community
@@ -68,7 +67,6 @@ describe Topic do
 
       expect(results).to eq [topic1, topic2, topic3]
     end
-
   end
 
   describe "notifications" do