Refactor settings model

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

app/models/concerns/settings/remote_storage_settings.rb

@@ -0,0 +1,16 @@
+module Settings
+  module RemoteStorageSettings
+    extend ActiveSupport::Concern
+
+    included do
+      field :remotestorage_enabled, type: :boolean,
+        default: ENV["RS_STORAGE_URL"].present?
+
+      field :rs_storage_url, type: :string,
+        default: ENV["RS_STORAGE_URL"].presence
+
+      field :rs_redis_url, type: :string,
+        default: ENV["RS_REDIS_URL"] || "redis://localhost:6379/1"
+    end
+  end
+end