Add basic SDG Management content section

Note using `params[:relatable_type].classify` is recognized as a
security risk by some tools. However, it's a false positive, since we've
added constraints to the URL so that paramenter can only have the values
we trust.

spec/routing/sdg_management_routes_spec.rb

@@ -0,0 +1,32 @@
+require "rails_helper"
+
+describe "SDG Management routes" do
+  it "maps routes for relatable classes" do
+    expect(get("/sdg_management/proposals")).to route_to(
+      controller: "sdg_management/relations",
+      action: "index",
+      relatable_type: "proposals"
+    )
+  end
+
+  it "admits named routes" do
+    expect(get(sdg_management_polls_path)).to route_to(
+      controller: "sdg_management/relations",
+      action: "index",
+      relatable_type: "polls"
+    )
+  end
+
+  it "routes relatable types containing a slash" do
+    expect(url_for(
+      controller: "sdg_management/relations",
+      action: "index",
+      relatable_type: "legislation/processes",
+      only_path: true
+    )).to eq "/sdg_management/legislation/processes"
+  end
+
+  it "does not accept non-relatable classes" do
+    expect(get("/sdg_management/tags")).not_to be_routable
+  end
+end