Use case-insensitive matching on email address

This patch addresses an issue where a client may have upper-case letters in
their email address. When visiting the client portal to check the ticket
status, previously, the exact same case would be required in the 'Email
Address' box.

This patch remove the case sensitivity for email logins.
Merge pull request #876 from greezybacon/issue/875

Use case-insensitive matching on email address

Reviewed-By: Peter Rotich <peter@osticket.com>