OEmbed support for PreviewCard (#2337)

* 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

spec/services/fetch_link_card_service_spec.rb

@@ -9,6 +9,6 @@ RSpec.describe FetchLinkCardService do
     status = Fabricate(:status, text: 'Check out http://example.中国')
 
     FetchLinkCardService.new.call(status)
-    expect(a_request(:get, 'http://example.xn--fiqs8s/')).to have_been_made
+    expect(a_request(:get, 'http://example.xn--fiqs8s/')).to have_been_made.at_least_once
   end
 end