Add Search Relevance/Usefulness Measurement

Created by: tistru

Request by Customer: https://app.hubspot.com/contacts/2762526/company/554275594

Feature request description

• Can sourcegraph incorporate some notion of tracking how long a user spent looking for a particular thing?
• Really like the idea of measuring view time for usefulness or relevance (Search ranking/relevance)

Use Case

• We're trying to come up with a measurable key result for improving developer productivity.
• While surveys are nice, they don't give a great picture as to how something like time-to-ship improves due to codesearch

Additional context

• Would equate this to something like "what if google returned all the results but had zero relevance?"
• Would be nice to have some sort of correctness vs usefulness measurement