Improve RTL in the walkthrough

[bhousel]

A few things going on here:

• Tooltip placement math ignores RTL
• The navigation buttons should support RTL (probably going to switch these to display:flex too)
• curtain, curtain-tooltip, and intro-nav-wrap should all be under #id-container, not body, in order to pick up the dir=rtl attribute

[bhousel]

curtain, curtain-tooltip, and intro-nav-wrap

• also all comboboxes created by d3.combobox.js should be under #id-container not body