Show plugins list on project integrations page

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

app/views/projects/settings/integrations/show.html.haml

@@ -3,3 +3,4 @@
 - page_title 'Integrations'
 = render 'projects/hooks/index'
 = render 'projects/services/index'
+= render 'shared/plugins/index'

app/views/shared/plugins/_index.html.haml

+- plugins = Gitlab::Plugin.files
+
+.row.prepend-top-default
+  .col-lg-4
+    %h4.prepend-top-0
+      Plugins
+    %p
+      #{link_to 'Plugins', help_page_path('administration/plugins')} are similar to
+      system hooks but can be are executed as files instead of URL.
+
+  .col-lg-8.append-bottom-default
+    - if plugins.any?
+      %ul.content-list
+        - plugins.each do |file|
+          %li
+            %span.title
+              = File.basename(file)
+    - else
+      %p.settings-message.text-center.append-bottom-0
+        No plugins found.