Bug 17381: Add system preference SCOMainUserBlock

Bug 17381: Add system preference SCOMainUserBlock

This patch adds a system preference SCOMainUserBlock to display custom HTML
on the self checkout page.

To test:
- Set up self checkout (see section Self Checkout in Circulaton sysprefs )
- Apply patch
- Update db
- Add some HTML system preference SCOMainUserBlock
- Go to sco page
- Verify that HTML displays on page (logged in and not logged in)

New version based on Koha.Preference TT plugin as of comment #7
Depends on Bug 12691

Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>