[sdesk-494] Name files in exported zip file based on slugling and unique_id of the story
[sdesk-494] Name files in exported zip file based on slugling and unique_id of the story

[sdesk-494] Flake styling changes

[sdesk-494] Remove all invalid chars for Windows OS filename

mdhaman/superdesk-core,marwoodandrew/superdesk-core,mugurrus/superdesk-core,mugurrus/superdesk-core,marwoodandrew/superdesk-core,petrjasek/superdesk-core,ioanpocol/superdesk-core,mdhaman/superdesk-core,ancafarcas/superdesk-core,nistormihai/superdesk-core,hlmnrmr/superdesk-core,superdesk/superdesk-core,superdesk/superdesk-core,hlmnrmr/superdesk-core,superdesk/superdesk-core,mdhaman/superdesk-core,superdesk/superdesk-core,ancafarcas/superdesk-core,nistormihai/superdesk-core,petrjasek/superdesk-core,petrjasek/superdesk-core,petrjasek/superdesk-core,mugurrus/superdesk-core,ioanpocol/superdesk-core,ioanpocol/superdesk-core