* Add tests

* Fix some link previews being incorrectly generated from different prior links

PR #12403 added a cache to avoid redundant queries when the OEmbed endpoint can
be guessed from the URL. This caching mechanism is not perfectly correct as
there is no guarantee that all pages from a given domain share the same
OEmbed provider endpoint.

This PR prevents the FetchOEmbedService from caching OEmbed endpoint that
cannot be generalized by replacing a fully-qualified URL from the endpoint's
parameters, greatly reducing the number of incorrect cached generalizations.

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