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/lib/activitypub/linked_data_signature_spec.rb

@@ -16,6 +16,10 @@ RSpec.describe ActivityPub::LinkedDataSignature do
 
   subject { described_class.new(json) }
 
+  before do
+    stub_jsonld_contexts!
+  end
+
   describe '#verify_account!' do
     context 'when signature matches' do
       let(:raw_signature) do