The link crawler expects HTML documents, so set the `Accept`
header accordingly.

Fixes #12618

Fix #12433

* add youtube oembed endpoint

* add check for oembed endpoint

* change unless for a more readable if

* clear blank lines

* endpoint via https

* Fix string literal in condition

* use cache for endpoints

* use cache for endpoints

* clean up and adding check

* clean up and remove redundant return

* add html check

* add false to return

* use double quotes

* use double quotes

* Clean up

* Fix undefined method error in sidekiq

Body can be not nil but still be empty, which causes a
`NoMethodError: undefined method `[]' for nil:NilClass` further in the
code. This checks for an empty body to avoid the issue.

* Fix codeclimate issue

* 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