Replace CRLF with LF when loading drafts

This is necessary because we save the offset and length of the user-
supplied text in the identity header. These values are then later used
to split the draft in user text and quoted message.
When calculating these values we operate on a string with LF line
endings. Ideally we want to do the reverse operation on the same
string, but when saving the message to the server LF is converted to
CRLF to create RFC-conforming messages.

This is only a hack and will probably be the cause of more trouble in
the future. A better solution would be to make the identity header more
robust or get rid of it entirely.