Demo instance features

To set up a demo instance (stackspin#1397 (closed)), we need a few features:

• add a form to the login page to create an (admin) user account given an email address;
• add a warning banner to that login page and to the dashboard itself saying that this is a demo instance, and all data is essentially public, and data gets removed every night.
• make those additions conditional, so they only apply to demo.stackspin.net.