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/post_status_service_spec.rb

@@ -1,6 +1,6 @@
 require 'rails_helper'
 
-RSpec.describe PostStatusService do
+RSpec.describe PostStatusService, type: :service do
   subject { PostStatusService.new }
 
   it 'creates a new status' do