Product Analytics MVP plan

Split and improve !27730 (closed) so it can make it into master.

What and why => #211568.

To summarize: product analytics as a feature for user apps with GitLab as warehouse and UI.

How it will work:

The feature is behind a feature flag and disabled by default. Enable on per-project level. Once enabled you see "Product Analytics" nav tab in project UI.

By navigating to "Product Analytics" you go to "Example" tab and copy showed JS script to your application. Your application will send async js requests to register every event to GitLab collector. After that events are displayed on project page UI.

TODO

Original merge request !27730 (closed). I decided to split it in smaller parts:

• Database support => !35168 (merged)
• Rails models + fixtures => !35869 (merged)
• Rails collector => !36443 (merged)
• Rails UI => !36693 (merged)
• Render few sample graphs based on events data => !37578 (merged)