UI Fixes for hideMenu and hideSidebar

[irichter]

This fixes issues I've discovered with the hideMenu=true and hideSidebar=true configurations:

• When hiding the menu, the calculation for the available canvas height assumed it was present anyway, leaving a white strip at the bottom of the window/frame.
• When hiding the sidebar, narrow screens would not display the main canvas because the CSS that hides the sidebar was selecting the <div> with the canvas instead.

See commit messages for how I fixed this.

[pfalstad]

Looks good, I will merge this. Thanks!