Add snowplow haml attr helper

What does this MR do?

• Adds SnowplowHelper with snowplow_attr method. This method accepts the tracking values and returns a data hash. It returns an empty hash if snowplow is disabled.

• Update import_project_page from https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/7403 to use the new attr to test.

• With snowplow enabled

• With snowplow disabled

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/7827

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