Add sentry JS

What does this MR do?

Adds sentry JS

EE port: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1667

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

Why was this MR needed?

We need to be able to monitor the health of our JS.

Screenshots (if relevant)

The top error is an ajax error (I know we already monitor the backend using sentry so any ajax errors should already be picked up, but I don't think that makes this useless).

The second error is a manual test.

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 merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

/cc @jschatz1