Add a username slug to the user URL

This way it won't be possible to browse all user URLs by just going to
/users/1, /users/2, /users/3, ... and collect usernames, which might not
be desirable in some cases.

Note we could use the username as a URL parameter and just find the user
with `@user = User.find_by!(id: id, username: username)`, but since
usernames might contain strange characters, this might lead to
strange/ugly URLs.

Finally, note we're using `username.to_s` in order to cover the case
where the username is `nil` (as is the case with erased users).

spec/controllers/users_controller_spec.rb

@@ -0,0 +1,39 @@
+require "rails_helper"
+
+describe UsersController do
+  describe "GET show" do
+    let!(:user) { create(:user, username: "James Jameson") }
+
+    it "finds a user by ID and slug" do
+      get :show, params: { id: "#{user.id}-james-jameson" }
+
+      expect(response).to be_successful
+    end
+
+    it "does not find a user by just an ID" do
+      expect do
+        get :show, params: { id: user.id }
+      end.to raise_error ActiveRecord::RecordNotFound
+    end
+
+    it "does not find a user by just a slug" do
+      expect do
+        get :show, params: { id: "james-jameson" }
+      end.to raise_error ActiveRecord::RecordNotFound
+    end
+
+    it "does not find a user with the wrong slug" do
+      expect do
+        get :show, params: { id: "#{user.id}-James Jameson" }
+      end.to raise_error ActiveRecord::RecordNotFound
+    end
+
+    it "finds users without username by ID" do
+      user.erase
+
+      get :show, params: { id: user.id }
+
+      expect(response).to be_successful
+    end
+  end
+end