Custom URLs in images without schema creates relative links

[mmtr]

Describe the bug
The custom URLs entered in an image block are not being sanitized / validated, so entering an URL without the schema (http(s)://) will create a relative URL.

To reproduce

1. Insert an image block and upload/select an image.
2. On the block toolbar, click on "Edit link".
3. Enter an address without http:// or https:// (i.e. wordpress.org).
4. Publish the page/post.
5. Visit the published post/page.

Expected behavior
To be taken to the external website upon clicking the image, but instead you're taken to an internal path (i.e. example.com/2019/08/23/my-post/wordpress.org).

[paaljoachim]

Here is another Image block issue. This is probably the 3 or 4th issue in relation to the Image block.

Pinging:
@WordPress/gutenberg-core

[paaljoachim]

Hi @mmtr Miguel

This issue looks like a duplicate of this:
Button Block does not prefix a URL with http://
#6841

The focus in the other issue is though the Buttons block. If you can leave a comment there then we can close this issue.
Thanks.

[mmtr]

@mrwweb already did that in #6841 (comment), so we can close this one as duplicate.