Preserve a readable masthead, even when a brower window is smaller

[jennaj]

One use case brought up at Galaxy Help (and that many have run into before): Tutorial and Galaxy both in open browser windows -- Galaxy's Masthead becomes messy when the window shrinks. GTN website creates a "hamburger" menu -- Galaxy could do the same.

Users can manage the panes open (right, middle, left). Or it could autoscale -- but that's a bigger ask. Keeping this simple.

Good

Good

Going

Gone

[mvdbeek]

Duplicate of #9844