Improve drafts management

[soywod]

Drafts management is really limited and not secure, a refactor is needed.

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email-lib

• Move local drafts from /tmp to XDG_DATA_HOME/himalaya/.
• Make drafts name unique so users can have multiple of them.
• Add option to encrypt local drafts.
• Add ability to "edit" a remote draft. Such feature does not exist on IMAP server, so it can technically be done by consuming (deleting) the targeted draft then to save it back.

himalaya

• Add ability to choose between multiple local drafts before editing.
• Add config option to enable local draft encryption.
• Add ability to edit a draft, then to either save or send it.

himalaya-vim

• Add ability to edit a draft, then to either save or send it.