Porting Modal UI to upstream

[Dylan Hardison]

I think porting the modal UI to upstream is a good thing to do. So I'm
doing that.
it's probably a lot of work. To that end, I've started a branch of the
current trunk to contain and share the effort.

Bug Modal specifically does not work with non-Mozilla (e.g. Sandstone) skins,
and when you force it to (which I have done) it looks bad. So on top
of this work, I've applied Ryan Wilson's Sandstone patch.

Note that I have allowed the upstream port of Bug Modal to allow other
skins to be selected.
Any skins that can't be made to work with it we should probably drop.

Note that with the .psgi support that has landed in trunk, it is
possible to get a local dev environment set up with only perl + cpanm.
I intend to write a blog post about this in short order.