Define and Implement Flow Metrics for SMS feature

[davismtl]

This card is to make sure that we have defined and then implemented our flow metrics for the SMS feature on email confirmation screen.

Without it, it may prove difficult to measure the success of phase 2, 3 and 4 of this feature.

[shane-tomlinson]

We are a ways through this at https://docs.google.com/document/d/133_Cu0TlWQZIw-ihMH1s-U_80I12MXZcvHQEuFhWbPg/edit#heading=h.pxrikbeif9zu

[shane-tomlinson]

@philbooth - would you say we've finished this?

[philbooth]

I would say so!

[davismtl]

Hey, I was just looking over the events.

Although not related to our flow metrics but related to the metrics to this feature, I added to the Feature document a list of amplitude metrics. (clicks, installs, install rate) We could potentially add retention to that list via Amplitude but I don't think it is part of the success metrics of feature.