serve: Rewrite of the Server-Sent Event code for build notifications. At the moment the server monitors the asset directories, and notifies the browser when an asset has changed and has been re-processed. * Fix issues around long-running requests/threads which mess up the ability to shutdown the server correctly with `CTRL-C` (see comments in code). * Move the notification queue to each SSE producer, to support having multiple pages open in a browser. * Add JS/CSS for showing quick notifications about re-processed assets. * Add support for hot-reloading CSS and pictures that have been re-processed.

# This is a utility module that can be used with any WSGI-compatible server
# like Werkzeug or Gunicorn. It returns a WSGI app for serving a PieCrust
# website located in the current working directory.
import os
from piecrust.wsgiutil import get_app


root_dir = os.getcwd()
app = get_app(root_dir)
# Add this for `mod_wsgi`.
application = app