WKS: fix handling of WKS v3 publishing request emails

v2 used to be an encrypted message with the application/vnd.gnupg.wks
metada inside, v3 is a multipart/mixed with text/plain part first and
the encrypted vnd.gnupg.wks subpart second. The second part is not marked
as an attachment or anything, so OTP ignores it and only shows the
text/plain part instead of our customized handler for the vnd.gnupg.wks
part.

To workaround the OTP behaviour, the plugin registers itself as a handler
for multipart/mixed and checks if the multipart contains text/plain and
application/vnd.gnupg.wks sub-parts in that order. If it does it skips the
text/plain part and only decodes the second one, otherwise ot behaves like
the native multipart/mixed parser and processes the first part.