Slightly reduce RAM usage (#7301)

* No need to re-require sidekiq plugins, they are required via Gemfile

* Add derailed_benchmarks tool, no need to require TTY gems in Gemfile

* Replace ruby-oembed with FetchOEmbedService

Reduce startup by 45382 allocated objects

* Remove preloaded JSON-LD in favour of caching HTTP responses

Reduce boot RAM by about 6 MiB

* Fix tests

* Fix test suite by stubbing out JSON-LD contexts

spec/services/subscribe_service_spec.rb

@@ -1,6 +1,6 @@
 require 'rails_helper'
 
-RSpec.describe SubscribeService do
+RSpec.describe SubscribeService, type: :service do
   let(:account) { Fabricate(:account, username: 'bob', domain: 'example.com', hub_url: 'http://hub.example.com') }
   subject { SubscribeService.new }