Track 'invited user' when they go through registration

What does this MR do and why?

Add tracking event to registration submit button and SSO button on /users/sign_up/ and /-/trial_registrations/new page to identify the registration type, in order to better identify invited user.

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.

How to set up and validate locally

1. Setup GDK to simulate SaaS.
2. Visit /users/sign_up and click the Register button and google SSO button.
3. In snowplow debugger extension (install if not present), observe the events
4. Visit /users/sign_up?invite_email=123@example.com and click the Register button and google SSO button.
5. Verify events in snowplow debugger extension
6. Visit /-/trial_registrations/new and click the Continue button and google SSO button.
7. Verify events in snowplow debugger extension

Related to #425950 (closed)