Add instrumentation to start a review button

We want to be able to track whether people are starting a review from the overview tab, or if they're adding comments/replies in the overview tab to an existing review.

Let's add some instrumentation to our buttons: https://docs.gitlab.com/ee/development/internal_analytics/internal_event_instrumentation/quick_start.html#data-track-attribute

Goal:

Be able to determine how frequently users comment/reply versus start a new review, versus add to an existing review from the overview tab. Inspired by #430123 (comment 1632322940)

Proposal

• Track clicks on "Add to review" on the overview page
• Track clicks on "Start a review" on the overview page
• Track clicks on "Add to review" on the changes page
• Track clicks on "Start a review" on the changes page