Tracking of custom events

What does this MR do?

This MR adds the ability to track custom events such as the number of Git pushes.

Are there points in the code the reviewer needs to double check?

The usual stuff.

Why was this MR needed?

We want to track more business level data such as the number of Git pushes, how many repositories are imported (and from where), etc.

What are the relevant issue numbers?

gitlab-org/gitlab-ce#13720

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together

cc @pcarranza