Add a basic script to "push" deploy

Currently updates are published by the "conservancy-www-update.sh" that does a
"pull" deploy with a 5-minutely job that runs on the web server. This doesn't
run `migrate`, `collectstatic` or restart the application, so certain types of
changes don't take effect, and even template changes often don't due to caching
template loader.

This script allows you to deploy more significant updates on-demand, but
requires SSH access.