Otherwise the fix will be overwritten by next _octicons.less update.

This is follow-up for 22b0dfbb35.

Semantic UI .icon 1em font-size has priority over .octicon 16px, resulting
octicons rendered at 14px font-size, which is not okay since Octicons are meant
to be shown sizes that are multiples of 16px.

- copy link button: #1396, #1168, #1668,
- synxtax highlight: #1712, #1549, #1315, #670
- z-index: #1942