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/fan_out_on_write_service_spec.rb

@@ -1,6 +1,6 @@
 require 'rails_helper'
 
-RSpec.describe FanOutOnWriteService do
+RSpec.describe FanOutOnWriteService, type: :service do
   let(:author)   { Fabricate(:account, username: 'tom') }
   let(:status)   { Fabricate(:status, text: 'Hello @alice #test', account: author) }
   let(:alice)    { Fabricate(:user, account: Fabricate(:account, username: 'alice')).account }