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/assets/javascripts/components/actions/cards.jsx

@@ -13,7 +13,7 @@ export function fetchStatusCard(id) {
     dispatch(fetchStatusCardRequest(id));
 
     api(getState).get(`/api/v1/statuses/${id}/card`).then(response => {
-      if (!response.data.url || !response.data.title || !response.data.description) {
+      if (!response.data.url) {
         return;
       }