Use a state machine for user status

The user status is a bit complex, since there are various states and
not all transitions between them make sense.

Using AASM means that we can name and restrict the transitions, which
hopefully makes them easier to reason about.

app/controllers/passwords_controller.rb

@@ -46,7 +46,7 @@ class PasswordsController < ApplicationController
         if params[:user]
           current_user.pass_crypt = params[:user][:pass_crypt]
           current_user.pass_crypt_confirmation = params[:user][:pass_crypt_confirmation]
-          current_user.status = "active" if current_user.status == "pending"
+          current_user.activate if current_user.may_activate?
           current_user.email_valid = true
 
           if current_user.save