Tim Heap's Wagtail Settings is a popular third party Wagtail app which makes it easy to manage site settings (e.g. social media accounts, Google Analytics keys, 'special' pages) within the admin interface. We've been using the app in most of our own Wagtail sites, and recommending it to others, so we asked Tim to contribute it to the core project. As a general rule, we recommend keeping site configuration in code, rather than the database, but we recognise that site administrators sometimes need to change settings outside of the git commit / test / deploy cycle.

Settings is a contrib app, so you need to add it to your 'INSTALLED_APPS' if you want to use it. Full docs here.