[FIX] Unlink abstract from submitter on deletion

also do not raise in getConference() for deleted abstracts
[FIX] confUser.py & confLogin.py handling error

- Closes #1239
[FIX] RegForm duplicate email does not raise error

- Closes #1264
[FIX] Category material deletion exception
[FIX] RegForm register as admin redirection

- Closes #1277
[FIX] Infinite recursion in poster sessions
[FIX] Use correct slot schedule for new poster sessions
[FIX] Unlink categs from avatars on delete