Marked synced back drafts as read
Marked synced back drafts as read

Summary: Previously we weren't adding the `\\Seen` IMAP flag to synced back drafts. This would make drafts appear as unread in Gmail.

Test Plan: Tested manually.

Reviewers: spang, mg

Reviewed By: mg

Projects: #sync-engine

Differential Revision: https://review.inboxapp.com/D1237

wakermahmud/sync-engine,wakermahmud/sync-engine,PriviPK/privipk-sync-engine,wakermahmud/sync-engine,nylas/sync-engine,closeio/nylas,nylas/sync-engine,jobscore/sync-engine,gale320/sync-engine,ErinCall/sync-engine,gale320/sync-engine,Eagles2F/sync-engine,nylas/sync-engine,PriviPK/privipk-sync-engine,closeio/nylas,closeio/nylas,EthanBlackburn/sync-engine,PriviPK/privipk-sync-engine,jobscore/sync-engine,PriviPK/privipk-sync-engine,PriviPK/privipk-sync-engine,wakermahmud/sync-engine,gale320/sync-engine,closeio/nylas,gale320/sync-engine,ErinCall/sync-engine,ErinCall/sync-engine,jobscore/sync-engine,EthanBlackburn/sync-engine,gale320/sync-engine,Eagles2F/sync-engine,Eagles2F/sync-engine,EthanBlackburn/sync-engine,jobscore/sync-engine,Eagles2F/sync-engine,EthanBlackburn/sync-engine,EthanBlackburn/sync-engine,wakermahmud/sync-engine,Eagles2F/sync-engine,ErinCall/sync-engine,ErinCall/sync-engine,nylas/sync-engine