Convert from CGI to WSGI

Instead of making admin.py and new_planet.py CGI scripts, set up a WSGI
handler (wsgi.py) and make that forward requests to admin.py and new_planet.py.
Although the added performance benefits of WSGI are not important for the
application, using WSGI results in more flexible deployment.
Stop using global objects to accumulate error messages

Since with the switch to WSGI, processes can persist and be shared between
requests, we can't use global variables for state - pass around error
message objects instead of using global objects.
Serve all content, even static content from a single WSGI handler

OpenShift, which we want to be able to deploy onto, is very limited in
it's handling of static content - only URLs that start with static/ can
be served statically. We can eventually migrate images, css, etc, there,
but for now, just serve static cotent directly out of the WSGI handler.

At a minimum, we should improve this by adding handling for If-Modified /
If-Match and/or by adding an expiration time.
Add setup.py and requirements.txt

Add information about the application needed for automatic
installation of dependencies.
Use an alternate directory structure for OpenShift

When running in an OpenShift environment, put writeable directories
in the (persistent) OpenShift data directory, rather in the
(wiped at each deployment) checkout root.
Stop copying skeleton directories

Since we now are handling references to static files through our
WSGI handler rather than directly, it's no longer necessary to
copy over the skeleton directory for each planet - we can simply
fall back to the skeleton at lookup time.

We also use this approach to properly handle layering of dynamic
and static content in the output directory - with the OpenShift
layout with a separate output directory, this is no longer
automatic.
requirements.txt: swap out uTidylib for pitidylib

uTidylib has a broken entry in pypi and hasn't been touched for 6
years or so - use pitidylib which should be a drop-in replacement
and is more maintained.
Handle index.html for subdirs

Don't just handle '/', handle index.html in the planet directories
as well.