Issue #13823: random message when all Todos are Done

## What does this MR do?

Shows a random message every time the To Do list is empty: https://gitlab.com/gitlab-org/gitlab-ce/issues/13823

## Are there points in the code the reviewer needs to double check?

It's my first contribution to GitLab so I might need some help:

 - I'm loading a yaml file in the controller. I'm not sure where would be the best place to put it.
 - I'm also having the error below running the tests but it doesn't seem related to my changes. I think I might have some issue with my config. Could you double check if the tests pass for you?

/home/pep/.rvm/gems/ruby-2.3.1/gems/actionpack-4.2.7.1/lib/action_dispatch/middleware/debug_exceptions.rb:21:in `call': No route matches [GET] "/autocomplete/users/sjobs@apple.com.json" (ActionController::RoutingError)

## Why was this MR needed?

https://gitlab.com/gitlab-org/gitlab-ce/issues/13823

## Screenshots (if relevant)

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [ ] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [ ] API support added
- Tests
  - [x] Added for this feature/bug
  - [ ] All builds are passing
- [x] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [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 it does - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/13823

See merge request !6818