Implement registration API

[urmade]

Description

1. Implement an API that takes an email address and an event ID. It should check if the email address is either new or verified.
2. If an email address is new or verified create a new registration ID and then write a new registration to an Azure Table storage. Send an email to the attendee with a boilerplate text, the registration ID and a QR code which is the encoded registration ID. In your response to the API caller, indicate if the mail was new or existing.
3. If the mail is new, it should also send an email to that person asking to verify their email address (from an user in Office, e.g. "Event Bot"). It then should write the mail address and a verification code to an Azure Table storage.
4. Implement an API to check if an email is already verified.
5. Implement an API to actually verify an email, using the verification code.
6. Implement an API to verify a registration. It takes a registration ID and checks if this ID is in the database. (Use case: In-Person ticket control)