Support twitter previews in Draftail embeds

Issue Summary

Draftail’s EmbedBlock have troubles displaying tweets’ informations (thumb / title)

Steps to Reproduce

1. Start a new project with wagtail start myproject
2. Add a RichTextField to an example model
3. Add a new content for this example model
4. Click embed block in draftail toolbar
5. Add a tweet https://twitter.com/libe/status/1110520362930974721?s=20

Expectations: get a sumary (title, description, thumbnail or any other relevant information) displaying in the textarea Result: Nothing except the generic look of embeds in the editor.

As discussed on slack with @thibaudcolas , this comes from here: https://github.com/wagtail/wagtail/blob/b1fab7084719ddbb88b9581567660930a6c082b4/client/src/components/Draftail/blocks/EmbedBlock.js#L13-L27

Also, as said on slack, on good generic workaround to this case (and other future cases like this one) could be to display embed source in the preview.

• I have confirmed that this issue can be reproduced as described on a fresh Wagtail project: yes

Technical details

• Python version: 3.7.2
• Django version: 2.1.7
• Wagtail version: 2.4

[thibaudcolas]

If anyone wants to finish this off, the only thing missing for #5545 to be merged is tests. There is guidance on the PR on what to write tests for exactly.

[thibaudcolas]

Done in #8167 😃