Rename lineHeight to lineHeightInPixels to distinguish from CSS value

CSS has a line-height property with values like '1.3', but we also have
a measured pixel value for line height. It's important to keep these
separate conceptually.
Re-measure the line height in pixels when the CSS line-height changes
Add specs for updating lineHeightInPixels when changing font size/family
Wire editor.lineHeight config value
Upgrade space-pen so $.fn.views doesn't return undefined values