Treat invitation email address case-insensitively

Currently, the invite module will treat two email addresses that are the same except for case as completely different. It allows a user to send multiple invitations to the same address if the case is different, and fails to detect a user registering with an invited email address unless it is entered in the same case as the invitation.

This patch always stores the invitation email address in lowercase, and when checking against the account email address uses the SQL "lower" function to do a case-insensitive match.