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

Gemfile

@@ -54,7 +54,7 @@ gem 'httplog', '~> 1.0'
 gem 'idn-ruby', require: 'idn'
 gem 'kaminari', '~> 1.1'
 gem 'link_header', '~> 0.0'
-gem 'mime-types', '~> 3.1'
+gem 'mime-types', '~> 3.1', require: 'mime/types/columnar'
 gem 'nokogiri', '~> 1.8'
 gem 'nsa', '~> 0.2'
 gem 'oj', '~> 3.5'
@@ -70,7 +70,6 @@ gem 'rails-settings-cached', '~> 0.6'
 gem 'redis', '~> 4.0', require: ['redis', 'redis/connection/hiredis']
 gem 'mario-redis-lock', '~> 1.2', require: 'redis_lock'
 gem 'rqrcode', '~> 0.10'
-gem 'ruby-oembed', '~> 0.12', require: 'oembed'
 gem 'ruby-progressbar', '~> 1.4'
 gem 'sanitize', '~> 4.6'
 gem 'sidekiq', '~> 5.1'
@@ -82,14 +81,14 @@ gem 'simple_form', '~> 4.0'
 gem 'sprockets-rails', '~> 3.2', require: 'sprockets/railtie'
 gem 'stoplight', '~> 2.1.3'
 gem 'strong_migrations', '~> 0.2'
-gem 'tty-command', '~> 0.8'
-gem 'tty-prompt', '~> 0.16'
+gem 'tty-command', '~> 0.8', require: false
+gem 'tty-prompt', '~> 0.16', require: false
 gem 'twitter-text', '~> 1.14'
 gem 'tzinfo-data', '~> 1.2018'
 gem 'webpacker', '~> 3.4'
 gem 'webpush'
 
-gem 'json-ld-preloaded', '~> 2.2'
+gem 'json-ld', '~> 2.2'
 gem 'rdf-normalize', '~> 0.3'
 
 group :development, :test do
@@ -135,6 +134,9 @@ group :development do
   gem 'capistrano-rails', '~> 1.3'
   gem 'capistrano-rbenv', '~> 2.1'
   gem 'capistrano-yarn', '~> 2.0'
+
+  gem 'derailed_benchmarks'
+  gem 'stackprof'
 end
 
 group :production do