Add some missing indices in signup_codes table

Summary:
We query the table by email and ip_address and expires_at columns.

I also replaced whereDate() with where() in these queries as more appropriate.

Test Plan: ./phpunit

Reviewers: #hkccp_developers, vanmeeuwen

Reviewed By: #hkccp_developers, vanmeeuwen

Subscribers: #hkccp_developers

Differential Revision: https://git.kolab.org/D3179