Bug 20685: Escape letter template content in textarea

Bug 20685: Escape letter template content in textarea

Test plan:
enter the following text into the Email template for 'HOLD_SLIP'
in Home › Tools › Notices & Slips › Modify notice

<div style='max-width: 4in;'>
<h3><<borrowers.surname>>, <<borrowers.firstname>></h3><br />
<<borrowers.streetnumber>> <<borrowers.address>><br />
<<borrowers.address2>><br />
<<borrowers.city>>, <<borrowers.state>> <<borrowers.zipcode>><br />
<br />
<br />
<img src="/cgi-bin/koha/svc/barcode?barcode=<<borrowers.cardnumber>>&type=Industrial2of5&notext=1">

Edit it again
Without this patch you will get:
<img src="/cgi-bin/koha/svc/barcode?barcode=<<borrowers.cardnumber>>&type=Industrial2of5¬ext=1">

Signed-off-by: Barton Chittenden <barton@bywatersolutions.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
(cherry picked from commit 305d3c36cb685fabce3ec4357ed769c25b12eb73)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>