Updated to handle errors correctly.
XDAT-3426 Case reopened after expected behavior of page was changed. Now all admins (along with project owners) can add and modify project user groups.
XNAT-3444 Added permissions check for all available modalities. Added caching of accessible groups to XDATUser.

xdat jar changes correspond to https://bitbucket.org/nrg/xdat_1_6dev/commits/f0a8fc218812746568ad0b2bf411c9a4100fe1e6
XNAT-3465 Added sorting to table before returning representation.
XNAT-3460 Added checks for project accessibility on both open (i.e. no study instance UID specified)
and targeted requests (i.e. study instance UID specified).
Added 'disabled' property to button object (to set a button as 'disabled' by adding 'disabled' attribute and 'disabled' class, hopefully there's no confusion about that).