When the button is clicked, the /preview endpoint is used to render a
preview text. The preview is inserted and the preview button is
replaced by an edit button to go back to edit mode. Alternatively, the
use can click on the preview to edit.
Some small CSS modifications are done to accomodate the
modification. Also, the preview is wrapped into `.isso-common
.text-wrapper .text` to not make the CSS more complex.
When in preview mode, the background is stripped/greyish in case it's not easy
to make a difference between preview and not preview (due to
unformatted text). We avoid to modify borders/shadow boxes because it
would make the design "jumpy".
Vincent Bernat