add Google Analytics tracking to to Sign In page (logged out side only)

Marketing is exploring ways that we can generate and end-to-end picture of user behavior, so that we can provide personalization and a better optimized path for user goals.

In the case of Sign In (as opposed to Sign Up - which has been approved) @systes has asked that a condition be met before we move forward with this, which is an analysis of past promises we've made to users about what we will and will not track.

• @dmor find the history of past promises and document in / link to this issue
• @dmor bring this back up to @sytses and other stakeholders for further discussion

FYI (with thanks!) @nicolasdular @s_awezec @justinfarris @hilaqu @tbarr @brodock @leipert @nwoods1