Initial website layout

I have just put https://debconf26.debconf.org/ up. It's using a sample crappy logo that I made, and a default theme. To improve things we need:

• A logo (static/img/logo.svg)
• a favicon based on the logo (static/img/favicon/favicon.svg + make -C static/img/favicon)
• pick a theme from https://bootswatch.com/ and change settings.py like so:

  diff --git i/settings.py w/settings.py
   index ccccebc..32b6c20 100644
   --- i/settings.py
   +++ w/settings.py
   @@ -23,7 +23,7 @@ root = Path(__file__).parent
    
    INSTALLED_APPS = (
        'dc25',
   -    'debconf.themes.cosmo',
   +    'debconf.themes.THEME',
        'news',
        'volunteers',
    ) + INSTALLED_APPS