Add SDG phase model

The purpose of this model will be to have different sections in the SDG
index.

spec/lib/tasks/db_spec.rb

@@ -9,27 +9,33 @@ describe "rake db:load_sdg" do
 
   it "populates empty databases and assigns targets correctly" do
     SDG::Goal.destroy_all
+    SDG::Phase.destroy_all
 
     run_rake_task
 
     expect(SDG::Goal.count).to eq 17
     expect(SDG::Target.count).to eq 169
     expect(SDG::Target["17.1"].goal.code).to eq 17
+    expect(SDG::Phase.count).to eq 3
   end
 
   it "does not create additional records on populated databases" do
     expect(SDG::Goal.count).to eq 17
     expect(SDG::Target.count).to eq 169
+    expect(SDG::Phase.count).to eq 3
 
     goal_id = SDG::Goal.last.id
     target_id = SDG::Target.last.id
+    phase_id = SDG::Phase.last.id
 
     run_rake_task
 
     expect(SDG::Goal.count).to eq 17
     expect(SDG::Target.count).to eq 169
+    expect(SDG::Phase.count).to eq 3
     expect(SDG::Goal.last.id).to eq goal_id
     expect(SDG::Target.last.id).to eq target_id
+    expect(SDG::Phase.last.id).to eq phase_id
   end
 end