LL-428 LL-429 LL-430
* impl possibility to check whether an error beside other errors from within a service operation is thrown (currently used for SSEntityDoesntExistErr errors)
* new service operations entityEntitiesAttachedGet and entityUserEntitiesAttach to have entities attached to other entities, e.g. for attaching files to a given discussion or one of its entries
* extending SSEntity with entities property
* extending discUserEntryAdd with entities parameter
* new table entities
* extended discUserWithEntriesGet and discsUserAllGet to retrieve also attached entities
* storing activities for discussion now independently (transaction-wise) from disc service call discEntryAdd
* re-factoring entity serv a little bit