Add Snowplow integration

Database checklist

When adding migrations:

• Updated db/schema.rb
• Added a down method so the migration can be reverted
• Added the output of the migration(s) to the MR body
• Added tests for the migration in spec/migrations if necessary (e.g. when migrating data)

General checklist

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Conforms to the code review guidelines
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• 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?
• End-to-end tests pass (package-and-qa manual pipeline job)

Migration Output

== 20180723134433 AddBasicSnowplowAttributesToApplicationSettings: migrating ==
-- add_column(:application_settings, :snowplow_enabled, :boolean, {:default=>false, :null=>false})
   -> 0.0085s
-- add_column(:application_settings, :snowplow_collector_uri, :string)
   -> 0.0005s
-- add_column(:application_settings, :snowplow_site_id, :string)
   -> 0.0006s
-- add_column(:application_settings, :snowplow_cookie_domain, :string)
   -> 0.0004s
== 20180723134433 AddBasicSnowplowAttributesToApplicationSettings: migrated (0.0102s) 

Screenshots

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ee/issues/6336