mailers: add procedure context to the confirmation link

This allows to redirect the user to the procedure they signed up for
even when the browser session is not available (like if they changed
of browser).

Fix #4738

app/controllers/users/confirmations_controller.rb

@@ -42,9 +42,19 @@ class Users::ConfirmationsController < Devise::ConfirmationsController
     if sign_in_after_confirmation?(resource)
       resource.remember_me = true
       sign_in(resource)
+    end
+
+    if procedure_from_params
+      commencer_path(path: procedure_from_params.path)
+    elsif signed_in?
+      # Will try to use `stored_location_for` to find a path
       after_sign_in_path_for(resource_name)
     else
       super(resource_name, resource)
     end
   end
+
+  def procedure_from_params
+    params[:procedure_id] && Procedure.find_by(id: params[:procedure_id])
+  end
 end

app/controllers/users/registrations_controller.rb

@@ -21,6 +21,12 @@ class Users::RegistrationsController < Devise::RegistrationsController
 
   # POST /resource
   def create
+    # We may need the confirmation mailer to access the current procedure.
+    # But there's no easy way to pass an argument to the mailer through
+    # all Devise code.
+    # So instead we use a per-request global variable.
+    CurrentConfirmation.procedure_after_confirmation = @procedure
+
     # Handle existing user trying to sign up again
     existing_user = User.find_by(email: params[:user][:email])
     if existing_user.present?