Add Snowplow support to backend

What does this MR do?

It adds Snowplow integration to backend. After this is merged, we can simply call Gitlab::SnowplowTracker.track_event to track events.

I also added NAMESPACE to the class and changed frontend to use it so that we don't have it on 2 places.

What are the relevant issue numbers?

Closes #8284 (closed)

Does this MR meet the acceptance criteria?

• [-] Changelog entry added, if necessary
• [-] Documentation created/updated via this MR
• [-] Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• [-] Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• [-] Conforms to the database guides
• [-] Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• 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?
• Security reports checked/validated by reviewer