Refactor settings model

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

app/models/concerns/settings/lightning_network_settings.rb

@@ -0,0 +1,25 @@
+module Settings
+  module LightningNetworkSettings
+    extend ActiveSupport::Concern
+
+    included do
+      field :lndhub_api_url, type: :string,
+        default: ENV["LNDHUB_API_URL"].presence
+
+      field :lndhub_enabled, type: :boolean,
+        default: ENV["LNDHUB_API_URL"].present?
+
+      field :lndhub_admin_token, type: :string,
+        default: ENV["LNDHUB_ADMIN_TOKEN"].presence
+
+      field :lndhub_admin_enabled, type: :boolean,
+        default: ENV["LNDHUB_ADMIN_UI"] || false
+
+      field :lndhub_public_key, type: :string,
+        default: (ENV["LNDHUB_PUBLIC_KEY"] || "")
+
+      field :lndhub_keysend_enabled, type: :boolean,
+        default: -> { self.lndhub_public_key.present? }
+    end
+  end
+end