Fix activation bug

See #21.  Activation bug caused by Middlebury email visiting urls before they get to the user.  Accounts were activated before the user clicked the link, so the activation url didn't work.

Now we allow multiple clicks of activation url, all redirecting to the login page.  Middlebury email "clicking" the url doesn't hurt the activation process, as it confirms that the account is real anyway.  Letting the activation link be clicked twice is just so users don't get an error.