Encode outgoing emails with quoted-printable encoding

See the support topic here: http://www.phpbb.com/community/viewtopic.php?f=46&t=2144011

It looks like some email software has trouble processing our 8bit transfer encoding for emails. It insists on re-encoding everything to quoted-printable, botching the messages in the process.

We should either encode everything to quoted-printable (this seems to be a fairly safe thing to do in theory, my only reservation would be that this is a behavior change for all boards in existence) or have both 8bit and quoted-printable code paths and provide a hidden or a visible option to switch between them.

AmigoJack appears to have already done most if not all of the work for the actual implementation here: http://www.phpbb.com/community/viewtopic.php?f=46&t=2144011&start=45#p13106980