* Do not default the format in ProviderDiscovery
The format should be determined when discovering, as it is in the current
implementation, and it is a flaw if it is not determined.
* Spec ProviderDiscovery
* OEmbed support for PreviewCard
* Improve ProviderDiscovery code failure treatment
* Do not crawl links if there is a content warning, since those
don't display a link card anyway
* Reset db schema
* Fresh migrate
* Fix rubocop style issues
Fix#1681 - return existing access token when applicable instead of creating new
* Fix test
* Extract http client to helper
* Improve oembed controller
Akihiko Odaki (@fn_aki@pawoo.net)