Declares several CSS variables for styling buttons throughout the app
and uses them.
Removes the concept of a 'tertiary' button -- primary and secondary have
the exact same styling, so all tertiary buttons are changed to secondary
buttons and the existing secondary button (only replyBtn) is changed to
a primary button.
Also removes some of the many ways to modify a button color and
background -- there is no need for mixins to specifically style various
attributes of the paper-button when the ability to apply any styles
directly to it (via @apply --gr-button) is already supported.
Change-Id: I19a4114764df80b06175032b228a6ec63b414089
6fa20d3b3d