Change the default 'UserStatus'

Add a 'STAFF' and a 'GUEST' value in 'UserStatus' enum then change 'ADMIN' to
'LOCAL_ADMIN'.
Return 'null' instead of an exception for enum

When a key doesn(t exist in the 'eesc.properties' file, the code shouldn't look
for the 'valueOf' for enum fields (UserStatus and GroupType) but return 'null'
if no key has been found.