showing alert always if user has unconfirmed email

For #778.
Hope this approach is okay. But there is a failing test in `SPEC=spec/controllers/pull_request_downloads_controller_spec.rb`.

```
 1) PullRequestDownloadsController POST 'create' returns http success
     Failure/Error: post 'create'
       Double :user received unexpected message :confirmed? with (no args)
     # ./app/controllers/application_controller.rb:40:in `unconfirmed_email_alert'
     # ./spec/controllers/pull_request_downloads_controller_spec.rb:19:in `block (3 levels) in <top (required)>'
```
Not sure how to fix this. :cry:.

Can we just add `allow(user).to receive(:confirmed?)` in it?