Link previews: sending url=undefined sometimes

[humitos]

Today I found that if you:

1. go to https://test-builds.readthedocs.io/en/full-feature/shot-scraper.html
2. hover on any of those 3 links
3. you get an error in the console

That's OK because they are external URLs and the backend should block them.

However, when taking a deeper look at the error, it seems the problem is at the JS level because the url= attribute sent to the backend API is undefined instead the real href from the HTML a tag.

The URL hit should be https://test-builds.readthedocs.io/_/api/v3/embed/?url=https://datasette.io/ and the error reported should be External domain not allowed. domain=datasette.io".

@zanderle can you take a look at this? 🙏🏼