Bug 35063: Convert SelfCheckInMainUserBlock system preference to HTML customization

Bug 35063: Convert SelfCheckInMainUserBlock system preference to HTML customization

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

To test you should have some content in the SelfCheckInMainUserBlock
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 SelfCheckInMainUserBlock is now stored there.
- The HTML customization entry form should offer
  SelfCheckInMainUserBlock 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 SelfCheckInMainUserBlock HTML customization and add unique
  content to the "fr-FR" tab.

- Log into the self check-in system and confirm that the
  SelfCheckInMainUserBlock 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
  "SelfCheckInMainUserBlock." 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>