Improve registrar area linked user switch

- Introduce profile
- Move linked users to profile
- Use PUT

#599

app/controllers/registrar/current_user_controller.rb

@@ -3,10 +3,16 @@ class Registrar
     skip_authorization_check
 
     def switch
-      new_user = ApiUser.find(params[:new_user_id])
-      sign_in(new_user) if new_user.identity_code == current_user.identity_code
+      raise 'Cannot switch to unlinked user' unless current_user.linked_with?(new_user)
 
+      sign_in(new_user)
       redirect_to :back, notice: t('.switched', new_user: new_user)
     end
+
+    private
+
+    def new_user
+      @new_user ||= ApiUser.find(params[:new_user_id])
+    end
   end
 end

app/controllers/registrar/profile_controller.rb

@@ -0,0 +1,17 @@
+class Registrar
+  class ProfileController < BaseController
+    skip_authorization_check
+
+    helper_method :linked_users
+
+    def show
+      @user = current_user
+    end
+
+    private
+
+    def linked_users
+      current_user.linked_users
+    end
+  end
+end