master issue for drafts functionality

[tomholub]

• re-enable drafts + unobtrusive way to request for pass phrase if missing for draft #888
• frequency of draft save timer to 5 seconds, ensure it runs when tapping back #972
• draft wrongly saved before any field is edited #887
• send email from draft #834
• App should use existing draft when creating new message in thread #965
• mark drafts in conversation with a red "draft" label #963
• render recipients instead of sender in list of drafts #873
• more crashes from 0.3.2 - when encrypting drafts #970

[tomholub]

Just adding, the issues / implementation don't have to be word-for-word as described above, please use your judgement, and also look at what Gmail does. Please share your notes here, as @DenBond7 is working on the same functionality and may be interested.