Add support for site favicons

[yuvipanda]

Currently we don't have a way to specify favicons for all pages. We should add this as a setting that takes a URL

Tasks

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1. Get the favicon from the community or Yuvi
2. Upload it appropriately
3. Add a new FAVICON_URL setting to settings.py that links to the path of the favicon file
4. Add a new favicon_url context processor to context_processors.py that will make this setting available to all of the pages
5. Have the template use this setting

Definition of done

• Django setting for specifying favicon is present
• Favicon is set for the cellmap challeng