Bug 11257: PREDUEDST should have more useful default text

Bug 11257: PREDUEDST should have more useful default text

This patch changes the default text of PEDUEDST to the following:
    Dear <<borrowers.firstname>> <<borrowers.surname>>,
    The following item(s) will be due soon:
    <<items.content>>

Note: I dont beleive ive done everything requesting in the bug, ie there
was a request to add  <<items.content>> to the documentation of
sample_notices.yml, but I couldn't find documentation for it. If there's
things I missed, please tell me so I can make the appropraite changes,
or feel free to make the changes yourself.

Test plan:
1. Select "edit" on PREDUEDST in the notices and slips page (Home -> tools
        -> Notices and slips)
2. Open "email", and note the default text.
3. Apply the patch, and peform a fresh install of Koha.
4. Navigate to the same page. The default text should have changed.

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
JD amended patch: Remove double spaces in PREDUEDST

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>