Show notifications button even if user is not member of a project

Notifications button was unavailable if user wasn't member of the
project, even if protected project is available via group privileges.

Showing disabled button with explanation tool-tip is less confusing.

This closes #2846.

app/controllers/projects_controller.rb

@@ -88,6 +88,7 @@ class ProjectsController < ApplicationController
           else
             if current_user
               @membership = @project.project_member_by_id(current_user.id)
+              @group_member = GroupMember.find_by(user_id: current_user.id)
             end
 
             render :show

app/views/projects/buttons/_notifications.html.haml

-- return unless @membership
+- return unless [@membership, @group_member].any?
 
-= form_tag profile_notifications_path, method: :put, remote: true, class: 'inline-form', id: 'notification-form' do
+- if @membership
+  = form_tag profile_notifications_path, method: :put, remote: true, class: 'inline-form', id: 'notification-form' do
     = hidden_field_tag :notification_type, 'project'
     = hidden_field_tag :notification_id, @membership.id
     = hidden_field_tag :notification_level
@@ -12,3 +13,9 @@
       %ul.dropdown-menu.dropdown-menu-right.project-home-dropdown
         - Notification.project_notification_levels.each do |level|
           = notification_list_item(level, @membership)
+
+- elsif @group_member
+  .btn.btn-new.disabled#notifications-button
+    = icon('bell')
+    = notification_label(@group_member)
+    = icon('angle-down')