Theme support

[stuartmaxwell]

Implement themes.

• [X] Uploaded to ./templates/djpress/{{ theme_name }}
• [X] Move the default theme to ./templates/djpress/default
• [X] Selected with a setting: DJPRESS_SETTINGS["THEME"] = "{{ theme_name }}"
• [X] If the configured theme doesn't exist, then fall back to default
• [X] If the configured theme doesn't exist, raise an error
• [x] Need to document the template loader order
• [X] Custom themes would need at least an index.html file
• [X] Static files…
  • [X] Themes could define any static resource, but the user would need to ensure it's copied to the correct static directory
  • [x] Would be better to have the theme and static files in one folder, e.g. ~~`./templates/djpress/{{ theme_name }}/static/css` etc.~~
  • [x] Custom static paths? Is this possible? Yes, a custom loader is possible
  • [X] Perhaps recommend copying files manually for now, e.g. ./static/djpress/{{ theme_name }}/css/styles.css etc. (avoid premature optimisation)

Enough details to get started...