Fix issue with endless redirect

What does this MR do and why?

In an effort to have a solid test suite around the existing registration flow, we'd like to backfill the test suite.

• Invite registration flow without tasks
• Invite registration flow with tasks

And fix issue with endless redirect and add feature tests for invitation with tasks

How to set up and validate locally

• Setup gdk to simulate SaaS - https://docs.gitlab.com/ee/development/ee_features.html#simulate-a-saas-instance and restart GDK
• Visit any group members page with such url: groups/<group-path>/-/group_members?open_modal=invite_members_for_task
• Invite new member. Copy invitation link from logs.
• Finish onboarding and make sure you can go to the root path.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #363540 (closed)

app/controllers/registrations/welcome_controller.rb

@@ -4,6 +4,7 @@ module Registrations
   class WelcomeController < ApplicationController
     include OneTrustCSP
     include GoogleAnalyticsCSP
+    include ::Gitlab::Utils::StrongMemoize
 
     layout 'minimal'
     skip_before_action :authenticate_user!, :required_signup_info, :check_two_factor_requirement, only: [:show, :update]
@@ -24,6 +25,7 @@ def update
 
       if result.success?
         track_event('successfully_submitted_form')
+        finish_onboarding_on_welcome_page unless complete_to_signup_onboarding?
 
         redirect_to update_success_path
       else
@@ -34,6 +36,8 @@ def update
     private
 
     def registering_from_invite?(members)
+      # If there are more than one member it will mean we have been invited to multiple projects/groups and
+      # are not able to distinguish which one we should putting the user in after registration
       members.count == 1 && members.last.source.present?
     end
 
@@ -61,33 +65,33 @@ def members_activity_path(members)
     end
 
     # overridden in EE
-    def redirect_to_signup_onboarding?
+    def complete_to_signup_onboarding?
       false
     end
 
-    def redirect_for_tasks_to_be_done?
-      MemberTask.for_members(current_user.members).exists?
+    def invites_with_tasks_to_be_done?
+      MemberTask.for_members(user_members).exists?
     end
 
     def update_success_path
-      if redirect_for_tasks_to_be_done?
-        finish_onboarding_on_welcome_page
-        return issues_dashboard_path(assignee_username: current_user.username)
-      end
-
-      return signup_onboarding_path if redirect_to_signup_onboarding?
-
-      finish_onboarding_on_welcome_page
-      members = current_user.members
-
-      if registering_from_invite?(members)
-        flash[:notice] = helpers.invite_accepted_notice(members.last)
-        return members_activity_path(members)
+      if invites_with_tasks_to_be_done?
+        issues_dashboard_path(assignee_username: current_user.username)
+      elsif complete_to_signup_onboarding? # trials/regular registration on .com
+        signup_onboarding_path
+      elsif registering_from_invite?(user_members) # invites w/o tasks due to order
+        flash[:notice] = helpers.invite_accepted_notice(user_members.last)
+        members_activity_path(user_members)
+      else
+        # Subscription registrations goes through here as well.
+        # Invites will come here too if there is more than 1.
+        path_for_signed_in_user(current_user)
       end
+    end
 
-      # subscription registrations goes through here as well
-      path_for_signed_in_user(current_user)
+    def user_members
+      current_user.members
     end
+    strong_memoize_attr :user_members
 
     # overridden in EE
     def finish_onboarding_on_welcome_page; end