[1.5.x] Fixed #8627 -- Prevented textareas to swallow first newline content

Browsers consider the first newline in textareas as some display
artifact, not real content. Hence they are not sending it back to
the server. If we want to keep initial newlines, we have to add one
when we render the textarea.
Thanks bastih for the report and initial patch.

Backport of 78f66691 from master.