Add image border in Markdown preview

## What does this MR do?

- add image border in Markdown preview like it is show after a comment is saved
- fix margin and shade of  `<hr>` in Markdown preview

## Are there points in the code the reviewer needs to double check?

Is `.md` the right class to use?

## Why was this MR needed?

The border was missing.

## What are the relevant issue numbers?

fixes #19652

## Screenshots

### Before

![before](/uploads/683e2af7278c33d29b2a1ae19d1136e3/before.png)

### After

![after](/uploads/07dcee531a5696303cb34b4a88357e8a/after.png)


See merge request !5162