Signup limits

Summary:
TODO:

• add index on signup_codes.ip_address and signup_codes.email columns
• make sure the stored email is lowercased?

Test Plan: ./phpunit

Reviewers: #hkccp_developers, machniak, vanmeeuwen

Reviewed By: #hkccp_developers, vanmeeuwen

Subscribers: mollekopf, #hkccp_developers

Differential Revision: https://git.kolab.org/D3149