(Bug 4196) New comments by replying to comment notifications

* Refactors the existing LJ::Emailpost into
 DW::EmailPost/DW::EmailPost::Base/DW::EmailPost::Entry/DW::EmailPost::Reply

* DW::EmailPost::Entry works by providing a PIN, same as before

* DW::EmailPost::Reply works by having a one-time auth per
 comment/comment thread, not time-limited, automatically generated by
 DW

* this contains the working backend, but the email output still needs an
 addition of a Reply-To, and new instructions, so we can take advantage
 of this functionality