Add combined groups/project GTM tracking

What does this MR do and why?

This MR closes gitlab-com/marketing/digital-experience/buyer-experience#361 (closed)

The combined group/project form at /users/sign_up/groups_projects/new has been rolled out, as per !67614 (merged). It's now the way that new users interact with this step of onboarding.

This MR adjusts the original Google Tag Manager spec to account for this new form. It adds an event to the submit event on the new form that will track an event called combinedGroupProjectFormSubmit.

Screenshots or screen recordings

video1601336088

How to set up and validate locally

1. Create a Google Tag Manager account and container ID. You should just need to follow the step 1 of the instructions. The GTM container can be empty - it just needs to exist.
2. In gitlab.yml, add your GTM ID in the extra block for both the google_tag_manager_id and google_tag_manager_nonce_id values. Make sure to restart GDK (or do this step prior to starting GDK) for settings to take effect.

   extra:
    google_tag_manager_id: 'GTM-YOURIDHERE'
    google_tag_manager_nonce_id: 'GTM-YOURIDHERE'

3. Start or restart your GDK
4. Turn on Feature.enable(:combined_registration)
5. Turn on Feature.enable(:gitlab_gtm_datalayer)
6. Open the Google Tag Assistant
7. Point the tag assistant to http://localhost:3000/users/sign_up and create a new account.
8. Step through the onboarding process until you get to /users/sign_up/groups_projects/new
9. Keep an eye on the dataLayer tab in the Tag Assistant
10. Submit the combined groups/projects form
11. Check for the combinedGroupProjectFormSubmit event in the dataLayer tab.

MR acceptance checklist

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