user upgrades: update upgrade landing page.

* Add a frequently asked questions section.
* Add nicer looking upgrade buttons.
* Format the page nicer.
* Prevent users from attempting invalid upgrades on users that are
  already Platinum or above.

app/policies/user_upgrade_policy.rb

@@ -3,6 +3,10 @@ class UserUpgradePolicy < ApplicationPolicy
     user.is_member?
   end
 
+  def new?
+    UserUpgrade.enabled?
+  end
+
   def show?
     record.recipient == user || record.purchaser == user || user.is_owner?
   end