Add email confirmation before signup

[PurkkaKoodari]

As discussed in #18, popular events often become F5 wars and may be affected by bots. One way to counteract issues would be to require confirming an email address before signup opens.

This would prevent the same person from reserving multiple spots. The email used for verification would not need to be the same as the email used for signup. The requirement could also be lifted after the first few seconds of a signup.

To be most effective, this verification would likely require aalto.fi addresses (hopefully configurable for cross-university events). This would need to be case sensitive and disallow + in the mailbox part to stop aliases.

While not terribly effective by itself against bots, this could be combined with (and a rather strict requirement for) entry randomization (#19). Especially when combined with it, the confirmation might only be required for signups before the randomization.