fix: Making buttons taller to match spec

A fix for the issue described in this ticket - #32 (closed) - which rightly points out that the buttons on staging looked a little cramped compared to the spec. (The linked ticket includes not only some helpful diagrams showing precise pixel dimensions and spacing, but also a list of which buttons the ticket applies to and which buttons are exceptions to the rule.)

Relevant padding and line-height values were adjusted in components/_button.scss to account for this change.

Closes: #32 (closed)