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