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/activitypub/fetch_remote_account_service_spec.rb

@@ -1,6 +1,6 @@
 require 'rails_helper'
 
-RSpec.describe ActivityPub::FetchRemoteAccountService do
+RSpec.describe ActivityPub::FetchRemoteAccountService, type: :service do
   subject { ActivityPub::FetchRemoteAccountService.new }
 
   let!(:actor) do

spec/services/activitypub/fetch_remote_status_service_spec.rb

@@ -1,6 +1,6 @@
 require 'rails_helper'
 
-RSpec.describe ActivityPub::FetchRemoteStatusService do
+RSpec.describe ActivityPub::FetchRemoteStatusService, type: :service do
   include ActionView::Helpers::TextHelper
 
   let(:sender) { Fabricate(:account) }

spec/services/activitypub/process_account_service_spec.rb

@@ -1,6 +1,6 @@
 require 'rails_helper'
 
-RSpec.describe ActivityPub::ProcessAccountService do
+RSpec.describe ActivityPub::ProcessAccountService, type: :service do
   subject { described_class.new }
 
   context 'property values' do

spec/services/activitypub/process_collection_service_spec.rb

@@ -1,6 +1,6 @@
 require 'rails_helper'
 
-RSpec.describe ActivityPub::ProcessCollectionService do
+RSpec.describe ActivityPub::ProcessCollectionService, type: :service do
   let(:actor) { Fabricate(:account, domain: 'example.com', uri: 'http://example.com/account') }
 
   let(:payload) do