Bug 35065: Convert SelfCheckHelpMessage system preference to HTML customization

Bug 35065: Convert SelfCheckHelpMessage system preference to HTML customization

This patch moves the SelfCheckHelpMessage system preference into HTML
customizations, making it possible to have language- and
library-specific content.

To test you should have some content in the SelfCheckHelpMessage
system preference before applying the patch. Apply the patch, run the
database update process, and rebuild the OPAC CSS.

- In the staff client, go to Tools -> HTML customizations and verify
  that the content from SelfCheckHelpMessage is now stored there.
- The HTML customization entry form should offer SelfCheckHelpMessage
  as a choice under "Display location."
- Update and reinstall active translations (for instance fr-FR):
  - perl misc/translator/translate update fr-FR
  - perl misc/translator/translate install fr-FR
- Enable the translation if necessary under Administration -> System
  preferences -> language.
- Enable the "opaclanguagesdisplay" preference if necessary.
- Edit the SelfCheckHelpMessage HTML customization and add unique
  content to the "fr-FR" tab.

- Log into the self checkout system and click the "Help" link at the
  top.
- On the help page, confirm that the SelfCheckHelpMessage content is
  shown there.
- Switch to your updated translation and confirm that the content you
  added for your translation shows up correctly.
- Go to Administration -> System preferences and search for
  "SelfCheckHelpMessage." It should return no results.

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>