Add auth pages

[jbrunton]

Mostly based on this tutorial: https://developer.mozilla.org/en-US/docs/Learn/Server-side/Django/Authentication

This adds:

• Very basic base template (using Semantic UI as a framework).
• An empty home page.
• A menu that has 'Sign-in' and 'Sign-out' links as appropriate.

Work in progress – by default Django reuses the templates for the admin site, which aren't appropriate for the end user. Need to add some more custom templates to replace those.