Fix dashboard UI bugs and more

[CL-Jeremy]

This PR fixes a few UI bugs I spontaneously encountered:

• Fixes emojis in repo titles getting head-cut and tail-cut in dashboard feed due to introduction of 1.25 em emojis at 1 em line-height, by simply using the original 1 3/7 em value of semantic.css
• Fixes regression (too long repo names should be capped to 70%) in Style and template tweaks #13828 due to flex children not respecting properties like overflow: hidden;, and removes a block of dead style code
• Follow-up to SVG Spacing restricted to direct descendants of items #14761, removes extraneous code for top navbar and correct right margin for Font Awesome
• Fixes color emphasis inversion in arc-green theme for top right buttons (edit, delete) on commit view boxes

[CL-Jeremy]

Just came to the idea to use the server octicon for admin settings. Looks pretty understandable to me. Will add if people like it.

[lunny]

Could you give some screenshots?

[CL-Jeremy]

[CL-Jeremy]

stylelint is complaining about the extra space here. Will fix this later. Waiting for reactions regarding switching to
Site Administration

[CL-Jeremy]

@noerw seems to be confused about what I meant to replace with the server icon. It's this button here. Do you think it's a good replacement to the triple gear icon?

Currently in this PR:

Proposal:

[CL-Jeremy]

Pushing for now so I won't forget about it if PR takes too long. Will revert last commit if people object the change.