Fix EZP-23195: An empty line at the beginning of a Text block will be lost

JIRA: https://jira.ez.no/browse/EZP-23195

# Description

As explained in the issue, if a text block value starts with a new line, this one will be lost after the form validation (actually when the form is displayed the second time). It seems like this is happening because [the HTML documents are parsed like a SGML document](http://www.w3.org/TR/html401/appendix/notes.html#notes-line-breaks) so the first line break is ignored. Oddly, the last line is not removed and I was not able to find any relevant info in the HTML5 specifications...

Note: the same is valid for PlatformUI and is fixed in https://github.com/ezsystems/PlatformUIBundle/pull/102

# Test

manual tests in Firefox, Chrome and IE10.
yes, that was my first idea but if we do that, we'll add new line at the end of the Text block after each edit.