This makes quotes created by user display properly in other MUAs like
Thunderbird and not start with space in MUAs that don't support format=flowed.
To distinguish user-created quotes from top-quote inserted by Delta
Chat, a newline is inserted if there is no top-quote and the first
line starts with ">".
Nested quotes are not supported, e.g. line starting with "> >" will
start with ">" on the next line if wrapped.
Sticky encryption rule, requiring that all replies to encrypted messages
are encrypted, applies only to messages with a quote now.
Co-Authored-By: B. Petersen <r10s@b44t.com>