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/lib/foodsoft_links.rb

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+require 'foodsoft_links/engine'

plugins/links/lib/foodsoft_links/engine.rb

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+module FoodsoftLinks
+  class Engine < ::Rails::Engine
+    def navigation(primary, context)
+      primary.item :links, I18n.t('navigation.links'), '#', if: Proc.new { visble_links(context).any? }  do |subnav|
+        visble_links(context).each do |link|
+          subnav.item link.id, link.name, context.link_path(link)
+        end
+      end
+      # move to left before admin item
+      if i = primary.items.index(primary[:admin])
+        primary.items.insert(i, primary.items.delete_at(-1))
+      end
+    end
+
+    def visble_links(context)
+      ret = Link.ordered
+
+      current_user = context.current_user
+      unless current_user.role_admin?
+        workgroups = current_user.workgroups.map(&:id)
+        ret = ret.where(workgroup: [nil] + workgroups)
+      end
+
+      ret
+    end
+  end
+end

plugins/links/lib/foodsoft_links/version.rb

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+module FoodsoftLinks
+  VERSION = "0.0.1"
+end