Get the list of documentation versions out of the database

Instead of having to update this list manually in multiple places when
releasing new versions, just take the information out of the database
where it has to be anyway.

Fixes #90
Closes #93

Also make the code automatically pick up wich PDF files exist in the
static checkout, and auto-detect their size, both A4 and US sizes. This
removes yet one more manual step, yay!

Fixes #163
Move docs templates into the docs/ app, now that they are not plain pages
Use same header in manual list and manual archive
Expire list of docs and manuals when versions are updated
Oops, we're not supposed to include supported versions in the archive
Fix links to frontpage of documentation for old versions

I forgot that old versions have strange names for their homepage, and
we only switched to index.html in 7.2.