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

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

Definition of done

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