Product line-wide consistency

[tjsavage]

• Make font application consistent across elements (Remove unneeded import pulling roboto font paper-toast#24)
• Make attribute vs. class usage consistent (paper-elements rfc specifications #23)

[notwaldorf]

Notes: for the font-consistency item, PolymerElements/paper-styles#80 should get us most of the way there. If all the elements would just use the mixins defined in typography.html, vs using their own, then changing the font across all paper elements is just a css selector

<style is="custom-style">
    :root {
      --paper-font-common-base: {
        font-family: serif !important;
      };
    };
</style>

[notwaldorf]

Going to close this and track this into a separate planning document for now.