Validating image url input and disabling media insert button when inp…

[pkwasnik]

…ut is empty

Suggested merge commit message (convention)

Fix (image, media-embed): Validating image url input and disabling media insert button when input is empty. Closes #7917.

---

Additional information

For example – encountered issues, assumptions you had to make, other affected tickets, etc.

[jodator]

Great!

I've changed one thing from the original code. I think that we should expose isEnabled as Boolean and we can to it easily with binding and custom callback (not directly stated in the docs).