Added a placeholder for not previewable HTML snippet.

[magda-chrzescian]

Suggested merge commit message (convention)

Fix (html-embed): An information should be displayed if the HTML snippet is not previewable. Closes #8435.

[magda-chrzescian]

@oleq Sinc it's totally different solution than the one we discussed, I'd like to wait for some initial review before writing the tests.

[magda-chrzescian]

The current state is as follows:
- "No preview available" is displayed for all invisible by design elements (scripts, head, comments, elements with display: none).
- Empty container is displayed if renderable, but invisible element is inside (div with 0x0 size, larger div with no background-color). This one may be a poor UX, but this is a rare case and it seems reasonable that if the user adds content that is invisible, he sees nothing.
- Proper information must be added if the snippet is empty,
- The content is displayed properly if an element in the HTML snippet is smaller than the placeholder behind it.

[magda-chrzescian]

This variable is added to be consistent with not merged yet https://github.com/ckeditor/ckeditor5/pull/8818/files, which also fixed the contrast ratio.