* 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

This preserves `<a ... class="u-url mention">` from other Mastodon instances.

Fix #2221 - Catch OpenSSL exceptions when loading remote avatars/headers/attachments
Don't strip "rel" attribute from <a> tags when sanitizing (microformats)

* Add target=_blank to user note

Open new window when click link from user profile in remote instance.

* fix rubocop

* 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