users: require email verification for signups from proxies.

Require users who signup using proxies to verify their email addresses
before they can perform any edits. For verification purposes, the email
must be a nondisposable address from a whitelist of trusted email
providers.

app/policies/application_policy.rb

@@ -39,7 +39,11 @@ class ApplicationPolicy
   end
 
   def unbanned?
-    user.is_member? && !user.is_banned?
+    user.is_member? && !user.is_banned? && verified?
+  end
+
+  def verified?
+    user.is_verified? || user.is_gold? || !user.requires_verification?
   end
 
   def policy(object)