Support proper en-/decoding of format=flowed

[s-ludwig]

Currently all outgoing messages are specified as format=flowed, but no line breaks are inserted and all received messages are not processed for flowed lines. Instead, flowed lines should be assembled to a single line to let the browser handle line wrapping. Sent messages in turn should be wrapped at 78 characters with soft breaks inserted.

Attention must be taken for signature lines "-- " and signed messages.