Refactor settings model

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

app/models/concerns/settings/general_settings.rb

@@ -0,0 +1,34 @@
+module Settings
+  module GeneralSettings
+    extend ActiveSupport::Concern
+
+    included do
+      field :primary_domain, type: :string,
+        default: ENV["PRIMARY_DOMAIN"].presence
+
+      field :accounts_domain, type: :string,
+        default: ENV["AKKOUNTS_DOMAIN"].presence
+
+      #
+      # Internal services
+      #
+
+      field :redis_url, type: :string,
+        default: ENV["REDIS_URL"] || "redis://localhost:6379/0"
+
+      field :s3_enabled, type: :boolean,
+        default: ENV["S3_ENABLED"] && ENV["S3_ENABLED"].to_s != "false"
+
+      field :sentry_enabled, type: :boolean, readonly: true,
+        default: ENV["SENTRY_DSN"].present?
+
+      #
+      # Registrations
+      #
+
+      field :reserved_usernames, type: :array, default: %w[
+        account accounts donations mail webmaster support
+      ]
+    end
+  end
+end