Resolve "Track onboarding events in Snowplow"

What does this MR do?

Adds the tracking to the user onboarding widget for the following:

• "Got it": ('trackStructEvent','onboarding','click_button','part_Y_step_X','got_it','')
• Skip this step: ('trackStructEvent','onboarding','click_link','part_Y_step_X','skip_this_step','')
• Restart this step: ('trackStructEvent','onboarding','click_link','part_Y_step_X','restart_this_step','')
• Exit Learn GitLab: ('trackStructEvent','onboarding','click_link','part_Y_step_X','exit_learn_gitlab','')

part_Y_step_X, where part_Y corresponds to parts 1 through 3 of the onboarding, and step_X is each step within each of the 3 sections of the "Learn GitLab" onboarding journey.

In addition, it adds the nav_source=onboarding param to the "Skip this for now" link on the onboarding welcome page. This way we can identify the redirected page accordingly.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Closes #11409 (closed)