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
-  = hidden_field_tag :notification_type, 'project'
-  = hidden_field_tag :notification_id, @membership.id
-  = hidden_field_tag :notification_level
-  %span.dropdown
-    %a.dropdown-new.btn.btn-new#notifications-button{href: '#', "data-toggle" => "dropdown"}
-      = icon('bell')
-      = notification_label(@membership)
-      = icon('angle-down')
-    %ul.dropdown-menu.dropdown-menu-right.project-home-dropdown
-      - Notification.project_notification_levels.each do |level|
-        = notification_list_item(level, @membership)
+- 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
+    %span.dropdown
+      %a.dropdown-new.btn.btn-new#notifications-button{href: '#', "data-toggle" => "dropdown"}
+        = icon('bell')
+        = notification_label(@membership)
+        = icon('angle-down')
+      %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')