Add empty SDG goal show page

Note we're using the code instead of the ID to get the goal in the URL.
IMHO this is what most people would expect; visiting a URL with a "7"
takes you to SDG number 7, and not to the one with "7" as a database ID.

In order to avoid tests (either automated tests or manual tests) passing
by coincidence due to the goal ID and the goal code being the same, I'm
shuffling the codes before entering them in the databse.

I've tried using `resolve` in the routes so the code is automatically
taken into account, but it doesn't work since `resolve` cannot be used
inside a namespace, and here we're within the `sdg` namespace.

spec/routing/sdg_routes_spec.rb

@@ -0,0 +1,19 @@
+require "rails_helper"
+
+describe "SDG routes" do
+  it "maps goals to their code" do
+    expect(get("/sdg/goals/1")).to route_to(
+      controller: "sdg/goals",
+      action: "show",
+      code: "1"
+    )
+  end
+
+  it "requires using the code instead of the ID" do
+    expect(get(sdg_goal_path(SDG::Goal[2].code))).to route_to(
+      controller: "sdg/goals",
+      action: "show",
+      code: "2"
+    )
+  end
+end