Create component variant for icon-only buttons

Buttons that contain only an icon should

• have equal width and height
• have text / svg fill color #707070 (gray-700) by default

(https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/20077#note_84619366)

• this needs to be documented on https://design.gitlab.com/components/buttons
• we need to introduce a new CSS and / or Vue component that simplifies this