Fix background images with special characters

Just like we did for budgets, we're doing the same thing in all the
places where we render background images attached by either regular
users or administrators.

This way we correctly render background images with characters like
brackets or quotes.

spec/helpers/images_helper_spec.rb

@@ -0,0 +1,13 @@
+require "rails_helper"
+
+describe ImagesHelper do
+  describe "#attached_background_css" do
+    it "adds quotes around the path" do
+      expect(attached_background_css("myurl")).to eq "background-image: url('myurl');"
+    end
+
+    it "escapes quotes inside the path" do
+      expect(attached_background_css("url_'quotes'")).to eq "background-image: url('url_\\'quotes\\'');"
+    end
+  end
+end