Introduce utility components

This follows up on @gtsiolis's proposal here: gitlab-org/gitlab-services/design.gitlab.com#380 (closed)

Over the last few months, we've added a number of components that are not defined by the Pajamas specification. Some examples are:

• Intersperse: https://gitlab-org.gitlab.io/gitlab-ui/?path=/story/base-intersperse--default
• Friendly wrap: https://gitlab-org.gitlab.io/gitlab-ui/?path=/story/base-friendly-wrap--default
• SPrintf (work in progress): !901 (merged)

Until now, those components were added to the base category which is the most appropriate, but we are going to create a new utility category and move these components into it because they aren't UI components per se, they act more as helpers and it would make sense to decouple them from the base category were we should find components that are defined by Pajamas.

/cc @dpisek