[lmi] PATCH: simplify attachment page handling

[Vadim Zeitlin]

 Hello,

 I'm a bit afraid of having conflicts in PDF generation code in the future,
as you're making sweeping changes to it while I'm trying to add pagination
functionality to it at the same time, so I'd like to start submitting parts
of my work even although it's not complete yet.

 The first one is rather trivial but I'm not sure what will you think about
it. On one hand, you might love it because it gets rid of a template and
makes the code simpler. OTOH you might disagree with it because it
sacrifices conceptual purity (it pretends that attachment page IS-A
numbered page, which is not really true) at the altar of convenience.

 The patch is at https://github.com/vadz/lmi/pull/84 and the commit message
there should, hopefully, describe it clearly. One thing I don't really say
in it is that the real motivation is that numbered_page::get_extra_pages()
will take place of pagination in the upcoming commits and so, if we want to
be able to paginate the attachment page as well (and I think we do; please
correct me if I'm wrong), this change is the most straightforward way to
implement it.

 Please let me know what do actually think of this patch, thanks in
advance,
VZ