Clean ajaxify styles

#### General cleaning of ajaxify cart styles
While working with a new theme, I noticed it was pretty difficult to override the ajax cart's styles. The drawer was defined with a dark colour scheme, and the modal was light. All colors were defined based on `#ajaxifyModal` or `#ajaxifyDrawer`, so overriding them was a specificity nightmare.

#### Ajax cart-related updates include:
- Prefix Sass mixin (for cleanliness)
- No more reliance on IDs for styles (except `#ajaxifyCart-overlay`)
- Modal and drawer styles are now the same
  - Styles are set with minimal Sass variables, which can be updated to match a store's theme settings or custom colours
- Removed general/unnecessary colors
- Moved the flip button styles below general cart styles
- Renamed some variables and mixins for less overlap and a cleaner naming convention
- Modal's overlay colour is now based on the modal's background

#### Other updates include:
- Renamed `$borderColor` to `$colorBorder` to match naming convention of other variables
- Added a background colour theme option

cc @shopify/fed @carolineschnapp 
:+1: