Unstick perf bar and move under navbar

What does this MR do?

Related issue: #118619

1. Make performance bar non-fixed, and move instead page layout so it scrolls with content.
2. Remove '.with-performance-bar` class, which lets us remove a few dozen CSS overrides

Why does this MR do that?

1. We often have bugs around top position for fixed/sticky elements due to the perf bar
2. Perf bar doesn't particularly need to be sticky. If you are someone who is reviewing performance halfway down the page, you are probably using browser devtools for it
3. Sticky headers are bad and cost valuable screen space

Screenshots (strongly suggested)

Screen_Recording_2021-02-19_at_3.47.14_pm