user invites: Log when users visit the sign-up page using the invited…

Created by: philipp-spiess

Closes #31613 (closed)

From the task:

Right now, when a user visits an invite link like https://sourcegraph.test:3443/sign-up?invitedBy=philippspiess we don't have anything logged that we can use to distinguish this from a visit to https://sourcegraph.test:3443/sign-up.

Test plan

You can enable the console debug mode (localStorage.enableLogDebug = "true" in the console) and verify that this emits an event. Note that this is not a "view" prefixed event since we already have a generic view event for all sign-up pages that won't allow us to add custom properties, unfortunately.