emails: add /emails index page.

Add emails index page at https://danbooru.donmai.us/emails. Mods can use
this page to view and search emails belonging to users below mod level.

app/policies/email_address_policy.rb

@@ -1,4 +1,8 @@
 class EmailAddressPolicy < ApplicationPolicy
+  def index?
+    user.is_moderator?
+  end
+
   def show?
     record.user_id == user.id || (user.is_moderator? && record.user.level < user.level)
   end