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.

db/migrate/20181203000000_create_links.foodsoft_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

db/schema.rb

@@ -235,6 +235,14 @@ ActiveRecord::Schema.define(version: 20181205010000) do
 
   add_index "invoices", ["supplier_id"], name: "index_invoices_on_supplier_id", using: :btree
 
+  create_table "links", force: :cascade do |t|
+    t.string  "name",                          null: false
+    t.string  "url",                           null: false
+    t.integer "workgroup_id"
+    t.boolean "indirect",      default: false, null: false
+    t.string  "authorization"
+  end
+
   create_table "mail_delivery_status", force: :cascade do |t|
     t.datetime "created_at"
     t.string   "email",           limit: 255,        null: false