Bug 1091295 - [Email] Failure to normalize newlines in composed messages. May cause some SMTP servers (ex: qmail) to refuse to send mail with a 451 error.

Note that r+-ing this is an implicit r+ of
https://github.com/mozilla-b2g/mail-fakeservers/pull/24
which I have speculatively already landed.  In that vein, note that I've also
updated our package.json to NOT use a tilde for mail-fakeservers since the
tilde regrettably defeats my whole concept of "we don't take the upgrade
without a gelam upgrade".
What about `/\r?\n|\r[^\n]/` for the regexp? Was a bit concerned that \r match on its own would match the \r\n case first, then we would see \r\n match, and it would end up as an extra \r\n. 

Although perhaps regexp greedy matching favors the longer match first and it is fine. That is likely the outcome, given that this does what you want. So, mainly just typing this for a "thinking out loud" thing.