SVG icon sizing and vertical alignment is cumbersome

Summary

While working on https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/18279 I had to vertically align icons myself within text buttons and even worse buttons. We should improve the developer experience when it comes to using icons, icons within buttons, buttons with different sizes, button groups.

Relevant code: app/assets/stylesheets/pages/projects.scss#L370-383

Links

Edited by Lukas Eipert