Add tracking of inbound traffic from browser extension/code host integration

Created by: erzhtor

Part 1 of https://github.com/sourcegraph/sourcegraph/issues/27082:

• Add a method of tracking to link created by the extension to Sourcegraph instances
• Add an event log to record an inbound link from the browser extension
• Ensure this event is present in Amplitude

Description

This PR:

• Adds (previously lacking) UTM markers to browser extension hover links
• Adds event log for browser extension UTM markers on web

This PR completes the following items from the issue above:

How to test

• Browser extenion
  • Run bext: sg run bext
  • Check that all links from browser extention to sourcegraph web have UTM markers

  • NOTE: UTM markers on hover links are added dynamically when clicking

  • NOTE: UTM markers are quickly trimmed on web after

• Web event logging
  • Start sourcegraph web: sg start web-standalone
  • Enable event log debugging: localStorage.setItem('eventLogDebug', true)
  • Open for example https://sourcegraph.test:3443/search?utm_source=chrome-extension
  • Check DevTools console that UTMCodeHostIntegration was logged w/ UTM props as argument and public_argument