Bug 32115: Add ID to check-out default help message dialog to allow customization

Bug 32115: Add ID to check-out default help message dialog to allow customization

This adds a div with the id attribute selfcheckdefaulthelpmessage to
the first paragraph of the web based self check help page.

To test:
* Enable WebBasedSelfCheck
* Go to: /cgi-bin/koha/sco/sco-main.pl in the OPAC
* Log in and open help
* Verify by looking at the source code, that the first paragraph
  starting with "If this is your first time..." now has an id.

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>