Add "preparing" status icon for jobs/pipelines
In https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/25586 a new preparing
state was added to the job lifecycle. It currently shares the icon with the created
state, but in order for the favicon(s) to display correctly it will need its own icon.
See https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/25586#note_147587825 for examples of what is required.