Add screen for email verification to start of onboarding flow

[markmhendrickson]

This entails adding a screen to the start of the onboarding flow for unauthenticated visitors to enter their email address and verify their contact before proceeding through the flow.

This will ensure that we can track real people through the onboarding experience and reduce the possibility of email identification confusion during the connection of services that may use varying email addresses for the same person.

This screen should direct the user to either click the link emailed to them or enter the code emailed to them (which depends on the completion of neotoma/sync-server#58).