Bug 18820: Main block size now adjust to OpacNavRight like it would for opacuserlogin

Bug 18820: Main block size now adjust to OpacNavRight like it would for opacuserlogin

Test plans  :
- "beforepatch.png" shows the result you get when OpacNavRight and opacnav are set to false.
- "opacuserlogin=f_opacnav=f.png" shows the result with the patch and OpacNavRight and opacnav set to false
- "opacuserlogin=t_opacnav=f.png" shows the result with the patch and OpacNavRight set to true and opacnav set to false

Applied patch, can confirm it functions as expected.
Signed-off-by: Dilan Johnpullé <dilan@calyx.net.au>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>