x/build/cmd/coordinator: unify header between build and status pages

[dmitshur]

The build dashboard is merging into the coordinator codebase in #34744, so maybe we should unify the header. It would help navigation between the build dashboard and the farmer status page, especially during local development.

We currently have:

And:

Perhaps something like this can work as a starting point.

CC @golang/release.

[dmitshur]

I came up with a simpler next step I'm happier with. Instead of adding a new "Coordinator" button, I'll just linkify the existing "Go Build Coordinator" text to take one to cmd/coordinator's "home" page.

simple.unified.header.mov

[gopherbot]

Change https://golang.org/cl/340116 mentions this issue: cmd/coordinator: unify headers visually