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/db/migrate/20181203000000_create_links.rb

@@ -0,0 +1,11 @@
+class CreateLinks < ActiveRecord::Migration
+  def change
+    create_table :links do |t|
+      t.string :name, null: false
+      t.string :url, null: false
+      t.references :workgroup
+      t.boolean :indirect, null: false, default: false
+      t.string :authorization
+    end
+  end
+end