RSM

 - [ZBX-4175] unify some translation strings;
make more messages translatable;
use doublequotes instead of squarebrackets to refer to objects;
HostGroup -> host group
cleanup whitespace around changed lines with an exception of api/classes/class.caction.php which was incredibly clean so deserved more work;
'ICMP Ping' -> 'ICMP ping' in server discovery notifications