Add links plugin

This can be used to link to external services related to the foodcoop.
With the indirect mode it is possible to implement a secure login to other
services. In that case Foodsoft will send a HTTP GET request and redirect
the user to the returned Location header. This allows the generation of
a one-time login URL.
A typical use-case would be that a workgroup, which is responsible for
the email account, does not need to share the login credentials and can
use a link within the Foodsoft instead.

plugins/links/app/models/link.rb

@@ -0,0 +1,5 @@
+class Link < ApplicationRecord
+  belongs_to :workgroup
+
+  scope :ordered, -> { order(:name) }
+end