users: let mods see email addresses on user profiles.

* Let Mods and Admins see the email addresses of users below their level.
* Let users see their own email address on their profile.
* Let users verify or edit their email address from their profile.

This is to make catching sockpuppets easier, and to make it easier for
users to fix their email.

app/policies/email_address_policy.rb

@@ -1,6 +1,6 @@
 class EmailAddressPolicy < ApplicationPolicy
   def show?
-    record.user_id == user.id
+    record.user_id == user.id || (user.is_moderator? && record.user.level < user.level)
   end
 
   def update?

app/policies/nil_class_policy.rb

@@ -0,0 +1,21 @@
+class NilClassPolicy < ApplicationPolicy
+  def index?
+    false
+  end
+
+  def show?
+    false
+  end
+
+  def create?
+    false
+  end
+
+  def update?
+    false
+  end
+
+  def destroy?
+    false
+  end
+end