Bug 10054 - When SingleBranchMode is enabled, allow superlibrarians to set logged...

Bug 10054 - When SingleBranchMode is enabled, allow superlibrarians to set logged in library

Currently, when SingleBranchMode is enabled, nobody can change the
logged in library in Koha. To change it requires disable
SingleBranchMode, switching the library, and re-enabling it. It seems
reasonable to allow superlibrarians to switch the logged in library even
if SingleBranchMode is enabled.

Test Plan:
1) Apply this patch
3) Enable SingleBranchMode
4) Log in as a superlibrarian
5) Attempt to change the logged in branch
   This change should succeed
6) Log in as a non-superlibrarian account
7) Attempt to change the logged n branch
   This should not be allowed, as per Koha's current behavior

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
I guess this doesn't do any harm even though the practical motivations
behind it, as described in the bug, seem otherwise solvable. If
SingleBranchMode is supposed to prevent librarians from switching their
logged in library why can they choose their library when they first log
in? Was SingleBranchMode supposed to only affect the OPAC originally? Or
should the login form be modified? This seems to be a band-aid on a
poorly-implemented feature.

Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
Patch works as decribed. As this only adds a check for a superlibrarian
it seems safe and should not interfere with normal staff accounts.
Signed-off-by: Jared Camins-Esakov <jcamins@cpbibliography.com>