Add vellum

- Stencil is now built on top of Vellum. 
- Fixed Alert component to work with Vellum
- Cleaned up styles in fixtures because we are using Vellum styles now

Status: Opened for Review
Reviewers: @kpeatt @jeffkamo @ry5n @avelinet
JIRA: ?
Is this necessary if we make the Vellum change to button/