Revert "Hopeful fix for activation redirect error on server."

This reverts commit f55ebfb49f4a9700d7e4f5a97973f81a847c4fe3.

Didn't actually fix anything. Problem was not in our code but in
the way Middlebury's email server handles emails.  See #21.
Fix account activation bug.

Account activation link can now be clicked multiple times and will
always activate the account and redirect to a login page with a
success message (django.contrib.messages).

We don't want to log users in automatically on activation if the
activation email works multiple times.  This would make the activation
link a permanent substitute for a password, which weakens security.

Closes #21.