Update search box component

The SearchBoxByType component is not following our design specs https://gitlab-org.gitlab.io/gitlab-design/hosted/design-gitlab-specs/search-spec-previews/

Screen_Shot_2019-10-18_at_6.11.09_PM

Note - currently confirming with UX if the icon 🔍 should be an optional prop.

Edited by Laura Montemayor