Fix admin API unconditionally requiring CSRF token (#17975)

Fixes #17898

Since #17204, the admin API has only been available through the web
application because of the unconditional requirement to provide a valid CSRF
token.

This commit changes it back to `null_session`, which should make it work
both with session-based authentication (provided a CSRF token) and with a
bearer token.

app/controllers/api/v1/admin/trends/links_controller.rb

@@ -1,8 +1,6 @@
 # frozen_string_literal: true
 
 class Api::V1::Admin::Trends::LinksController < Api::BaseController
-  protect_from_forgery with: :exception
-
   before_action -> { authorize_if_got_token! :'admin:read' }
   before_action :require_staff!
   before_action :set_links