Feat: component styles using CSS Modules

This MR is a follow up on the gitlab-ui Working Group's decision on moving components' styles over from gitlab-ce to gitlab-ui and initialise the structuring work for further fight against CSS bleeding, etc.

Premises:

• for now, only <gl-avatar> component has been switched to CSS Modules for this PoC phase. The rest of the components are untouched.
• variables.scss and mixins.scss have been directly copied from gitlab-ce
• avatar.scss has been "borrowed" ;) from @pgascouvaillancourt and @ealcantara from their great work on structuring components' styles