Log cloud UTM campaign entrypoints as a standalone event

Created by: philipp-spiess

Fixes #29483 (closed)

In order to better understand the effects of different cloud marketing campaigns, we want to log these UTM parameters when a link is clicked.

Based on the spreadsheet in #29483 (closed) we look for all UTM campaigns with utm_source thatequal hubspot and utm_capaign that start with cloud-onboarding-email. Including on-premise events wouldn't do anything as those events are not sent to analytics tools and we'd have to build a custom aggregation on the on-prem instances.

⚠️ Note: The UTM params are tracked as the public arguments (the third arggument of the log method) which should include them in our analytics data.

Test Plan

• Go to https://sourcegraph.test:3443/search?utm_source=hubspot&utm_campaign=cloud-onboarding-email6&utm_source=somethign&utm_term=learn-how-to-do-that-here&utm_content
• Enable log debugging (localStorage.setItem('eventLogDebug', true) in the console).
• Verify that the new event is logged: