MKT-820 -- Stop storing PII on cookies

[MKT-820](https://optimizely.atlassian.net/browse/MKT-820)

Instead of using email address with Segment.io's identify function, this PR uses either
a) `unique_user_id` when we have it
b) a random string

@kylerush could you give this a good look?
Yup, I agree with this.