* Whitelist allowed classes for federated statuses
Allowed classes are currently:
- Any microformats class (h/p/u/dt/e-*)
- the classes mention, hashtag, ellipses and invisible.
this last one is somewhat suspect, but Mastodon currently uses it to render hidden link text.
resolved#3790
* Fix code style
* 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
