Implementing Trial Discover Page Experiment Stage 2

What does this MR do and why?

Resolves https://gitlab.com/gitlab-org/gitlab/-/issues/412715

Implementing Trial Discover Page Experiment Stage 2

This MR finishes implementing the trial_discover_page experiment, which was started in !142824 (merged).

This MR adds the content for the /-/groups/<group_name>/discover page, and adds the experiment tracking.

The (long) list of tracking events are documented on the issue here.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Setup GDK to simulate SaaS
2. Setup and run Customers Dot
3. Enable feature flag by running echo "Feature.enable :trial_discover_page" | gdk rails c

Free Registration (My company)

1. Create a user at http://gdk.test:3000/users/sign_up
2. On http://gdk.test:3000/users/sign_up/welcome, select My company or team under Who will be using GitLab?
3. Finish creating user
4. Confirm user is on trial
5. Confirm Learn about features link is on trial widget and popup
6. Confirm http://gdk.test:3000/groups/<group_name>/-/discover renders page

Trial Registration

1. Sign out and navigate to http://gdk.test:3000/-/trial_registrations/new
2. Follow the trial registration flow and create a user
3. Confirm user is on trial
4. Confirm Learn about features link is on trial widget and popup
5. Confirm http://gdk.test:3000/groups/<group_name>/-/discover renders page

Finally

1. Confirm change isn't visible with feature flag disabled
2. Confirm existing users on a trial are excluded from the experiment
3. Confirm clicking the trial widget redirects to /discover page, instead of the billing page
4. Disable feature flag by running echo "Feature.disbale :trial_discover_page" | gdk rails c

Related to #412715