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

app/lib/provider_discovery.rb

@@ -0,0 +1,36 @@
+# frozen_string_literal: true
+
+class ProviderDiscovery < OEmbed::ProviderDiscovery
+  include HttpHelper
+
+  class << self
+    def discover_provider(url, options = {})
+      res    = http_client.get(url)
+      format = options[:format]
+
+      raise OEmbed::NotFound, url if res.code != 200 || res.mime_type != 'text/html'
+
+      html = Nokogiri::HTML(res.to_s)
+
+      if format.nil? || format == :json
+        provider_endpoint ||= html.at_xpath('//link[@type="application/json+oembed"]')&.attribute('href')&.value
+        format ||= :json if provider_endpoint
+      end
+
+      if format.nil? || format == :xml
+        provider_endpoint ||= html.at_xpath('//link[@type="application/xml+oembed"]')&.attribute('href')&.value
+        format ||= :xml if provider_endpoint
+      end
+
+      begin
+        provider_endpoint = Addressable::URI.parse(provider_endpoint)
+        provider_endpoint.query = nil
+        provider_endpoint = provider_endpoint.to_s
+      rescue Addressable::URI::InvalidURIError
+        raise OEmbed::NotFound, url
+      end
+
+      OEmbed::Provider.new(provider_endpoint, format || OEmbed::Formatter.default)
+    end
+  end
+end