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/features/admin/proposals_spec.rb

@@ -10,7 +10,6 @@ describe "Admin proposals" do
                   "admin_proposal_path"
 
   context "Index" do
-
     scenario "Search" do
       create(:proposal, title: "Make Pluto a planet again")
       create(:proposal, title: "Build a monument to honour CONSUL developers")
@@ -49,11 +48,9 @@ describe "Admin proposals" do
       within("#proposal_#{proposal.id}") { expect(page).to have_link "Select" }
       expect(proposal.reload.selected?).to be false
     end
-
   end
 
   context "Show" do
-
     scenario "View proposal" do
       create(:proposal, title: "Create a chaotic future", summary: "Chaos isn't controlled")
 
@@ -105,6 +102,5 @@ describe "Admin proposals" do
       expect(find_field("Mark as selected")).not_to be_checked
       expect(proposal.reload.selected?).to be false
     end
-
   end
 end