Update pipeline/build state icons

From https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5222#note_13087229. Since that merge request has already shipped, we should continue the discussion here.

There's some debate whether the new spinner icon for running and the clock for pending are good enough.

Personally, I really like having all the states have a consistent, circular shape, so here's a proposal for revising all of the icons.

Even at small sizes, I think these work.