Open invite members for task modal in Learn GitLab project

What does this MR do and why?

This is an experiment where we test the link Invite your colleagues on the Learn GitLab project:

• control: a link to an explanation issue
• candidate: a trigger to open the invite members for task modal

The hypothesis is that the candidate results in more members being added to a namespace.

Issue: gitlab-org/growth/team-tasks#459 (closed)

UX Issue: gitlab-org/growth/team-tasks#445 (closed)

Screenshots or screen recordings

Initial state	Link clicked	Colleagues invited

Quick demo of the experiment: https://youtu.be/gJl1Xf7G_Ts

How to set up and validate locally

1. IN rails console, enable the experiment

   Feature.enable(:invite_for_help_continuous_onboarding)

2. Register a new user and visit the newly created Learn GitLab project (http://localhost:3000/${group}/${project}/-/learn_gitlab)
3. Click the Invite your colleagues link

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.