Merge branch 'use-cached-project-count-ce' into 'master'

Used cached value of project count to reduce DB load

Following !5746, backport a change from https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/637/diffs#diff-2.

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- Tests
  - [x] All builds are passing
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !5754
Signed-off-by: Rémy Coutable <remy@rymai.me>

CHANGELOG

@@ -3,6 +3,7 @@ Please view this file on the master branch, on stable branches it's out of date.
 v 8.10.5
   - Add a data migration to fix some missing timestamps in the members table. !5670
   - Revert the "Defend against 'Host' header injection" change in the source NGINX templates. !5706
+  - Cache project count for 5 minutes to reduce DB load. !5746 & !5754
 
 v 8.10.4
   - Don't close referenced upstream issues from a forked project.

app/views/admin/dashboard/index.html.haml

@@ -108,7 +108,7 @@
           %h4 Projects
           .data
             = link_to admin_namespaces_projects_path do
-              %h1= number_with_delimiter(Project.count)
+              %h1= number_with_delimiter(Project.cached_count)
             %hr
             = link_to('New Project', new_project_path, class: "btn btn-new")
       .col-sm-4