Bug 35048: Convert SCOMainUserBlock system preference to HTML customization

Bug 35048: Convert SCOMainUserBlock system preference to HTML customization

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

The patch also makes a minor fix to CSS in order to get the "Back to
top" link to show up correctly in self-checkout and self-checkin.

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

- Log into the self checkout system and confirm that the
  SCOMainUserBlock 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
  "SCOMainUserBlock." It should return no results.

Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>