Add social icons to footer

[pmahnke]

Done

• added svg icons until Vanilla pattern is released
• updated —watch to use auto-prefixer

QA

• download branch
• ./run serve
• look at the footer on any page
• icons should link to ubuntu designers twitter, instagram and blog

Issue / Card

Fixes #17

Screenshots

[webteam-app]

Starting demo at: http://design.ubuntu.com-pr-89.run.demo.haus/

[kwm14]

@pmahnke until I look at the pattern for Vanilla.

Could you update to this sprite so it fits more inline with what we currently have across sites.

Update sprite https://assets.ubuntu.com/v1/e1c22739-design-ubuntu-social-sprite.svg

[webteam-app]

Starting demo at: http://design.ubuntu.com-pr-89.run.demo.haus/

[pmahnke]

@kwm14 ready for re-review

[kwm14]

Looking good 👍