Allow attachments to be ordered
Enhance ordering with a sprinkling of JavaScript

I've gone for the simplest thing possible, rather than attempting to
re-use the sortable.js code, which seems heavily tied to markup and
the specifics of ordering organisations and priorities. If I have the
time, I'll refactor the other sorting code so everything uses
something more generic.
UI tweaks to attachment ordering
Restrict ordering to editions that support it
Don't preload attachments

The preloading of attachments was causing the MySQL filter tests to
because of a bug where the order clause on the association is used
when loading the join model. Short of ripping out the join model
(which we are planning to do soon), I couldn't think of another way
to get this particular test failure to pass. I think we can live with
this as the MySQL document filter is only used in development/test
environments.