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?

DESCRIPTION

This change makes it so that we are not using PII (personally identifying information) as a unique identifier to our tracking platforms. There are two types of leads this affects: leads with an account and leads without an account.

Leads without an account

These leads will submit a form that does not create an account, like contact sales or the non-profits form. Since the form does not create an account we do not have a non-PII unique identifier for the user so we generate a random string. This random string allows us to track the user's behavior on the website after submitting the form so we can see what they did after submitting the contact sales form. Without the unique identifier, the lead's actions after submitting the form would not be tied to the lead.

Leads with an account

Obviously a random string is not ideal so in the case that the lead actually has an account we are using the new `unique_user_id` property that is generated in the product for all accounts. This is not a PII attribute of the lead's account since by looking at the value you would have no idea who it is. This allows us to attribute any actions the lead takes after signing in or creating an account to the given lead.