Support button groups

[jamesmockett]

What are you changing?

• Adds collapseUntil option to Inline layout component to allow collapsing into a single column below a given breakpoint.
• Horizontally centre button content by default. (This only has an affect when a button is stretched beyond its intrinsic default width.)

Why?

• This is to enable support for button groups where buttons are laid out horizontally at their default intrinsic width on larger viewports, but stretch to full width and stack horizontally on smaller viewports.

Images

Below tablet breakpoint

At tablet breakpoint