Resolve "Create a generic event listener for tracking clicks on GitLab.com"

What does this MR do?

Creates a generic JS function that in order to enable tracking of arbitrary events via snowplow.

Adds click tracking to the project creation page on GitLab.com, see https://gitlab.com/gitlab-org/gitlab-ee/issues/7429

Once imported, the tracking function can be called like:

Stats.trackEvent(category, eventName, { label, property, value });

CE port: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22002

What are the relevant issue numbers?

• https://gitlab.com/gitlab-org/gitlab-ee/issues/7424
• https://gitlab.com/gitlab-org/gitlab-ee/issues/7429

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?

Closes #7424 (closed)