Self-host snowplow.js for Snowplow analytics tracking

As described in https://gitlab.com/gitlab-org/gitlab-ee/issues/6329, we'd like to use Snowplow for event tracking on GitLab.com.

For JavaScript tracking, Snowplow relies on a script sp.js. While Snowplow offers a hosted version of this script on Cloudfront, we should self-host this so we're not reliant on third party hosted JS.

Please see https://github.com/snowplow/snowplow/wiki/self-hosting-snowplow-js. Specifically, we'd likely add this to our own pipeline, as described in the advanced options:

Add the un-minified snowplow.js into your own asset pipelining process and serve it using something other than CloudFront

We'll include this script in our trackers, planned in https://gitlab.com/gitlab-org/gitlab-ee/issues/6336.