Refactor settings model

Move the various sections to their own concerns, so they're easier to
find and maintain

app/models/concerns/settings/mastodon_settings.rb

@@ -0,0 +1,16 @@
+module Settings
+  module MastodonSettings
+    extend ActiveSupport::Concern
+
+    included do
+      field :mastodon_public_url, type: :string,
+        default: ENV["MASTODON_PUBLIC_URL"].presence
+
+      field :mastodon_enabled, type: :boolean,
+        default: ENV["MASTODON_PUBLIC_URL"].present?
+
+      field :mastodon_address_domain, type: :string,
+        default: ENV["MASTODON_ADDRESS_DOMAIN"].presence || self.primary_domain
+    end
+  end
+end