Summary:
Adds Google Tag Manager snippet to all login pages, and a new user
preference, recordSignUpEvent, that's set to true on first sign-in. The
client now checks for this preference, and if true, dynamically loads
Google Tag Manager to record a sign-up event. Afterwards, it removes
the preference.
Test Plan: Tested manually.
Reviewers: dsagal
Reviewed By: dsagal
Subscribers: dsagal
Differential Revision: https://phab.getgrist.com/D3319
