FIX: Properly associate user_profiles background urls via upload id.

`Upload#url` is more likely and can change from time to time. When it
does changes, we don't want to have to look through multiple tables to
ensure that the URLs are all up to date. Instead, we simply associate
uploads properly to `UserProfile` so that it does not have to replicate
the URLs in the table.

spec/serializers/user_serializer_spec.rb

@@ -41,13 +41,11 @@ describe UserSerializer do
   end
 
   context "with a user" do
-    let(:user) { Fabricate.build(:user, user_profile: Fabricate.build(:user_profile)) }
+    let(:user) { Fabricate(:user) }
     let(:serializer) { UserSerializer.new(user, scope: Guardian.new, root: false) }
     let(:json) { serializer.as_json }
-
-    it "produces json" do
-      expect(json).to be_present
-    end
+    let(:upload) { Fabricate(:upload) }
+    let(:upload2) { Fabricate(:upload) }
 
     context "with `enable_names` true" do
       before do
@@ -69,23 +67,15 @@ describe UserSerializer do
       end
     end
 
-    context "with filled out card background" do
+    context "with filled out backgrounds" do
       before do
-        user.user_profile.card_background = 'http://card.com'
+        user.user_profile.upload_card_background(upload)
+        user.user_profile.upload_profile_background(upload2)
       end
 
       it "has a profile background" do
-        expect(json[:card_background]).to eq 'http://card.com'
-      end
-    end
-
-    context "with filled out profile background" do
-      before do
-        user.user_profile.profile_background = 'http://background.com'
-      end
-
-      it "has a profile background" do
-        expect(json[:profile_background]).to eq 'http://background.com'
+        expect(json[:card_background_upload_url]).to eq(upload.url)
+        expect(json[:profile_background_upload_url]).to eq(upload2.url)
       end
     end