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

lib/tasks/mastodon.rake

@@ -2,6 +2,8 @@
 
 require 'optparse'
 require 'colorize'
+require 'tty-command'
+require 'tty-prompt'
 
 namespace :mastodon do
   desc 'Configure the instance for production use'