add mongo index to published.translation_id
add mongo index to published.translation_id

we use it to send notifications and it did slow down
the publishing

SDESK-6363

superdesk/superdesk-core,petrjasek/superdesk-core,superdesk/superdesk-core,superdesk/superdesk-core,petrjasek/superdesk-core,petrjasek/superdesk-core,superdesk/superdesk-core,petrjasek/superdesk-core