Implement account restoration

Currently, if a user deletes their account, they have no way to restore it. They also cannot create a new account with the same email. We should give them a way to restore their account and give instruction in the flash message that's shown when they try to login and/or signup with the same email. Maybe also include a link somewhere near the signup and login forms.

Facebook handles by just un-deleting your account if you log back in. This seems like a nice, simple way to handle it. If we go this route, we'll have to get Devise's forgot password functionality to work with soft-deleted users.