Report: clearer iconography in Perf metrics and Fancier Stuff

[ebidel]

The warning icon might be good for things in the Fancier stuff section that fails. The current red X is a bit harsh for these. We can keep the green checkbox when the audit passes.

For the perf metrics section, the current icons also don't make sense. Green check for "you've added user timing metrics". Yay! We could use the info icon and warning icon for this section since it's mostly informative and not scored.

[wardpeet]

can only say that this would be great :)