Adds styling to the toast component

What does this MR do?

This MR styles the toast component according to design specs:

https://design.gitlab.com/components/toasts/

https://gitlab-org.gitlab.io/gitlab-design/hosted/design-gitlab-specs/toasts-spec-previews/

Closes: gitlab-ui#237 (closed)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Code review guidelines
• Merge request performance guidelines
• Style guides

Performance and testing

• Tested in all supported browsers