Remove global styles

[fbarl]

It doesn't feel right that this modular components repo also contains global styles that we're implicitly expecting the end user to import.

The remaining global styles here consist of:

• _mixins.scss - these are not consumable by styled components and as such, they should probably be moved to the scope and service-ui repos (the only mixin still used seems to be material-shadow in service-ui repo)
• _reset.scss - most of those styles (if not all) can be moved to the theme, for the rest it's probably ok to assume the users are going to define them themselves