Feature/moar buttons

# Buttons
-----

- Buttons sizing has been seriously affected here. This adds versatility and just makes more sense. However, it will royally screw existing buttons
- add `.btn-md` (this will be the same as `.btn` as a default size)
- Convert existing `buttons.css.scss` to a DRYer, more Sassy way of generation. This is more versatile, if new button styles/sizes are ever desired
- `$btn-violet` is now used for tertiary button styles instead of secondary buttons (now `$btn-blue-soft`)